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Astrobiology:AVeryShortIntroduction

ASTROBIOLOGY

AVeryShortIntroduction

DavidC.Catling

Contents
Acknowledgements
Listofillustrations
1Whatisastrobiology?
2Fromstardusttoplanets,theabodesforlife
3Originsoflifeandenvironment
4Fromslimetothesublime
5Life:agenome’swayofmakingmoreandfittergenomes
6LifeintheSolarSystem
7Far-offworlds,distantsuns
8Controversiesandprospects
Furtherreading
Index

Acknowledgements

IthankProfessorJohnArmstrong,DrRoryBarnes,ProfessorJohnBaross,Dr

BillyBrazelton,ProfessorRogerBuick,DrRachelHorak,ImeldaKirby,and

ProfessorWoodySullivanforreadingpartsofthemanuscriptandoffering

variouscorrectionsandsuggestions.ThanksalsotoLathaMenonandMimi

Southwood,whoreadtheentiremanuscript,thelatterofferingtheperspectiveof

anon-scientist.Numerousstudentswhoattendedmyastrobiologyclassesatthe

UniversityofWashingtonovertheyearsalsohelpedmementallypreparefor

writingthisbook.AtOxfordUniversityPress,IthankEmmaMaandLatha

Menonfortheirencouragementandassistance.

Listofillustrations
1TheHertzsprung–Russelldiagram
Adaptedfrom‘StellarEvolutionandSocialEvolution:AStudyinParallelProcesses’(2005),Social
Evolution&History.4:1,136–59),reproducedwithpermissionofProfessorRobertCarneiro
2Left:Cross-sectionoftheworld’soldestfossilstromatolitesRight:Aplan
viewofthebeddingplaneofthestromatolites
PhotographsbyDavidC.Catling
3Theapproximatehistoryofatmosphericoxygen
Author’sowndiagram
4a)Schematicofprokaryote(archaeaandbacteria)versuseukaryotestructure;
b)Twobacteriacaughtintheactofconjugation
b)Credit:CharlesC.BrintonJr.andJudithCarnahan
5Left:DNAconsistsoftwostrandsconnectedtogether.Right:Inthree
dimensions,eachstrandisahelix,sothatoverallwehavea‘doublehelix’
6Theclassificationschemeformetabolismsinterrestriallife
7The‘treeoflife’constructedfromribosomalRNA
8a)ValleynetworksonMars;b)OutflowchannelRaviVallis
a)ESA/DLR/FUBerlin
(G.Neukum);b)NASA/JPL/Caltech/ArizonaStateUniversity

9TheGalileanmoonsofJupiter:Io,Europa,Ganymede,andCallisto

NASA/JPL/DLR

10a)AnetworkofchannelsthatappeartoflowintoaplainneartheHuygens

landingsite;b)ImageofthesurfaceattheHuygenslandingsite.

CourtesyofESA/NASA/UniversityofArizona

11Brainandbodymassforsomedifferentmammals.

AdaptedfromComparativeBiochemistryandPhysiologyPartA:Molecular&IntegrativePhysiology,

Vol.136:4,Hassiotis,M.,Paxinos,G.,andAshwell,K.W.S.,‘Theanatomyofthecerebralcortexofthe

echidna(Tachyglossusaculeatus)’,827–50.Copyright(2003),withpermissionfromElsevier

Chapter1

Whatisastrobiology?

Behindthename

‘Whatthehellisastrobiology?’anAmericanSecretServiceagentcriedintohis

walkie-talkie.Hehadjustbeencheckingtheidentityofanacademicvisitorto

NASA’sAmesResearchCenter,nearSanFrancisco.Thevisitorhadsaidthathe

wasattendingNASA’sfirstastrobiologyscienceconference.Ameshasan

airstripthatprovidesasecurelandingsiteforAirForceOne,and,inApril2000,

PresidentBillClintonhadjustflownintovisittheSanFranciscoBayarea,

bringingalonghisSecretServiceentourage.

Theagent’squestionwasafairone.Itwasonlyinthelate1990sthatascientific

consensusemergedaboutthemeaningofastrobiology.FewlaymenorSecret

Serviceagentswouldhaveheardoftheterm.Backthen,NASAbeganto

promotearesearchprogrammeinastrobiologyledbyAmes,whereIwas

workingasaspacescientist.Atfirst,someofmycolleaguesdislikedtheliteral

Greekmeaningofthe‘biologyofstars’.Onenotedwithascoffhowlife

couldn’texistinsidetheinfernosofstars.Alesscurmudgeonlyinterpretationis

thatthe‘astro’inastrobiologyconcernslifearoundstars,includingtheSun,or

simplylifeinspace.Infact,manyastrobiologistsareasmuchconcernedwith

thehistoryoflifeonEarthaswithlifeelsewhere.Astrobiologistsagreethatwe

shouldhaveafirmunderstandingofhowlifeevolvedonEarthinorderto

pondertheexistenceoflifeinouterspace.Yetoneoftheastonishingaspectsof

modernscienceisthatithassofarfailedtoanswerquestionsaboutbiologythat

evenachildmightask.HowdidlifeonEarthgetstarted?Wehavesomeideas

butthedetailsareunknown.WhichspecialpropertiesoftheEarthandtheSolar

Systemmakeourplanethabitable?Again,somethoughtsbutthereisstillmuch

tolearn.Andwhatcausedlifetoevolveintocomplexorganismsinsteadof

remainingsimple?Again,we’reuncertain.

Tofilltheseholesinhumanknowledge,astrobiologyhasemergedasabranchof

scienceconcernedwiththestudyoftheoriginandevolutionoflifeonEarthand

thepossiblevarietyoflifeelsewhere.Thisismyownpreferreddefinition.NASA

hasdefinedastrobiologyasthestudyoftheorigins,evolution,distribution,and

futureoflifeintheuniverse.Othercommondefinitionsarethestudyoflifeinthe

universeorthestudyoflifeinacosmiccontext.Withinthispurview,

astrobiologistspursuethequestion‘What’sthehistoryandfutureofterrestrial

life?’aswellas‘Istherelifeelsewhere?’

Fourdevelopmentscoincidedwiththeemergenceofastrobiologyasadiscipline

inthelate1990s.In1996,controversialsignsofancientlifeweredescribed

withinaMartianmeteorite—a1.9kilogrampieceofrockthathadbeenblasted

offthesurfaceofMarsbyanasteroidimpactandhadeventuallylandedin

Antarctica.Whethertheinterpretationoffossilizedmicroscopiclifewascorrect

ornot(seeChapter6),itsetpeoplethinking.Furthermore,overthepreceding

twodecades,biologistshadestablishedthatsomemicrobesnotonlytolerateda

muchlargerrangeofenvironmentsthanhadpreviouslybeenthoughtbut

actuallythrivedinextremesoftemperature,acid,pressure,orsalinity.Soit

becameplausibletocontemplateextraterrestrialmicrobesexistinginseemingly

hostileplaces.Athirdfindingcamein1996frompicturestakenbyNASA’s

Galileospacecraftoftheice-coveredsurfaceofJupiter’smoon,Europa,which

revealedpiecesoficethathaddriftedapartinthepast,suggestinganocean

belowanicycrust.Then,fromthemid1990sonwards,astronomersfound

increasingnumbersofextrasolarplanetsorexoplanets,whichareplanets

orbitingnotourSunbutotherstars.Thepossibilitythatlifemightresideon

exoplanetsorinthecosmicbackyardofourownSolarSystemprovidedan

impetusforaskingwhetherlifemightbecommonintheuniverse.

Astrobiologyinthehistoryofideas

Althoughastrobiologycametotheforeinthe1990s,thequestionofwhether

we’realoneintheuniversegoesbackmillennia.Thales(c.600BC),often

regardedasthefatherofWesternphilosophy,espousedtheideaofapluralityof

worldswithlife.Subsequently,theGreekatomistschoolfromLeucippusto

DemocritusandEpicurus,whichbelievedthatmatterwasmadeofindivisible

atoms,favouredsuch‘pluralism’.Metrodorus(c.400BC),afollowerof

Democritus,wrote,‘Itisunnaturalinalargefieldtohaveonlyonestalkof

wheat,andintheinfiniteuniverseonlyonelivingworld’.Butitwouldbe

incorrecttoequatetheancientphilosophers’pluralismwithourmodern

conceptionoflifeonMarsorexoplanets.Metrodorushadnocluethatstarswere

Sun-likeobjectsatenormousdistancesandbelievedthattheyformeddailyfrom

moistureintheEarth’satmosphere.Thepopulatedworldsoftheatomists’

imaginationwerebodiesinanintangiblespace,similartomodernideasof

paralleluniverses.Inanycase,theopposingviewofPlato(427–347BC)and

Aristotle(384–322BC)ultimatelydominated.TheirbeliefthattheEarthwas

uniquelyinhabitedandatthecentreoftheuniverseprevailedforoverathousand

years.

Eventually,RenaissanceastronomersshowedthattheEarthorbitedtheSun.

WiththerealizationthattheEarthwasmerelyanotherplanet,speculationssoon

aroseaboutextraterrestriallifeonotherplanetsintheSolarSystem.Johannes

Kepler(1571–1630),theGermanastronomerresponsibleforastronomy’sthree

lawsofplanetarymotion,happilyentertainedtheideaofinhabitedplanets.Then,

bytheendofthe17thcentury,theDutchastronomerChristiaanHuygens(1629–

95)wasimagininglifebeyondtheSolarSysteminhisbookCosmotheoros

(1698):‘allthosePlanetsthatsurroundthatprodigiousnumberofSuns.They

musthavetheirplantsandanimals,nayandtheirrationalcreaturestoo.’

Extraterrestriallifewassoinvoguethatin1755thephilosopherImmanuelKant

(1724–1804)wroteofintellectualsonJupiterandamorousVenusians.

Incontrast,somescholarswithreligiousviewscontinuedtoclingtoEarth’s

uniqueness.AnexamplewasCambridgeUniversity’sWilliamWhewell(1794–

1866),whoseOfthePluralityofWorlds(1853)arguedagainstotherinhabited

planetsinasortofforerunnerofacontemporarydebatecalledtheRareEarth

HypothesisthatIdiscussinChapter8.

Bythelate19thcentury,theissueoflifeelsewherewasseenasapurely

scientificmatter,thoughscienceitselfsoondevelopedsomeblindalleys.

TelescopicobservationsbyGiovanniSchiaparelli(1835–1910)andPercival

Lowell(1855–1916)createdasurgeofinterestinthepossibilityofintelligent

lifeonMars.Unfortunately,Lowell’sbeliefthathesawcanalsonMarswasan

opticalillusioncreatedwhenthemindconnectsdotsinblurryimages,andhis

ideasofMartiancivilizationswerefantasy.Increasingly,asastronomers

employedpainstakingtechniquessuchasexaminingspectraoflightfrom

planets,itbecameapparentthatthephysicalconditionsonvariousSolarSystem

planetsmightnotbesofavourableforlifeafterall.Thependulumswungso

firmlyintheotherdirectionthatbythemid20thcenturyfewastronomerswere

interestedinplanets.IttooktheSpaceAgetorekindleoldcuriosities.

Whilethebasicquestionsofastrobiologyareancient,theterm‘astrobiology’

onlysurfacedfromtimetotimebeforebecomingcommoninthe1990s.In1941,

anessayentitled‘Astrobiology’byLaurenceLafleur(aphilosopheratBrooklyn

College,NewYork)describedthewordmorenarrowlythanitsmodern

incarnationastheconsiderationoflifeotherthanonEarth.OttoStruve,an

astronomerattheUniversityofCaliforniainBerkeley,alsousedthetermin

1955todescribethesearchforextraterrestriallife.ARussianastrophysicist,

GavriilTikov(1875–1960),andaGermanastronomer,JoachimHerrmann,

publishedbooksentitledAstrobiologyin1953and1974,respectively,covering

popularideasofextraterrestriallife.

Themodernuseof‘astrobiology’wasintroducedin1995byWesHuntress,then

atNASA’sheadquartersinWashingtonDC.Atthetime,NASAscientistsargued

thatastudyoflifeoverscalesfromthemicrobialtothecosmicwasessentialfor

understandinglifeintheuniverse.Huntresslikedthewordastrobiologyforthis

aspirationandthenamestuck.

Infact,astrobiologywasreallyareinventionandexpansionofexobiology,a

fieldthatgoesbackseveraldecades.In1960,JoshuaLederberg(1925–2008)

coinedthewordexobiologyfor‘theevolutionoflifebeyondourownplanet’.

Lederberg,aNobelPrizewinnerfordiscoveriesinbacterialgenetics,arguedthat

anessentialpartofspaceexplorationshouldbesearchingforlife.Then,fromthe

1960sonwards,NASAfollowedLederberg’sadviceandfinancedexobiology

research.Butexobiologysoondevelopeditscritics.In1964,GeorgeGaylord

Simpson,aHarvardbiologist,quippedthat‘this“science”hasyetto

demonstratethatitssubjectmatterexists’.

Theadvantageoftoday’sastrobiologyisthatitcannotbeheldtoSimpson’s

chargebecauseitincludesthestudyoftheoriginandevolutionoflifeonEarth

atitscore.Astrobiologyalsoemphasizestheoriginandevolutionofplanetsasa

contextforlife,andsoembracesastronomersmorefirmlythanexobiology.

Exobiologyisnottheonlytermsimilartoastrobiology.Since1982,astronomers

haveofficiallyusedbioastronomyfortheastronomicalaspectsofthesearchfor

extraterrestriallife,whileearlierthewordcosmobiologywasfavouredbyJ.

DesmondBernal(1901–71),aninfluentialIrish-bornBritishphysicalchemist.

Butneitherofthosetermshasbecomewidespread.

Whatislife?

Astrobiologyraisesthedifficultquestionofhowtodefinelife.Whatisitexactly

thatwearelookingforbeyondEarth?Acommonapproachistolistlife’s

characteristics,whichincludereproduction,growth,energyutilizationthrough

metabolism,responsetotheenvironment,evolutionaryadaptation,andthe

orderedstructureofcellsandanatomy.Unfortunately,thiswayofdefininglifeis

unsatisfactoryforacoupleofreasons.First,thelistdescribeswhatlifedoes

ratherthanwhatlifeis.Second,mostoftheseaspectsoflifearenotunique.Life

hasstructuralordersuchascells,butsaltcrystalsarealsoordered.Someofmy

friendshavenochildrenbutthey’realive,Ithink,asaremulesthatcannot

reproduce.Growthanddevelopmentapplytolivingentities,butalsoto

spreadingfires.Alllifemetabolizesbutsodoesmycar.Lifereactstoits

environment,butamercurythermometeralsorespondstoitssurroundings.

Alternatively,somescientiststrytodefinelifeusingthermodynamics,thatis,

heatandenergyandtheirrelationshiptomatter,bysuggestingthattheessenceof

lifeisthepresenceofstablestructures,suchascellsandgeneticmaterial,

alongsideentropyproducedbymetabolicwasteandheat.

Thetermentropyrequiressomeclarification.Desperateteacherssearchingfora

quickanddirtyexplanationhaveoftencalledit‘disorder’.Entropyisnot

disorderbutanexactmeasureofenergydispersalamongstparticles,bethey

atomsormolecules.Energydispersesspatiallyandsotheenergyofgroupsof

particlesthatmovetogether,whichissaidtobecoherentenergy,candissipate.

Thus,abouncingballcomestorestbecauseitscoherentenergyofmotionis

convertedintoincoherentthermalmotionofmoleculesandatomsthrough

friction.Incontrast,astationaryballneverspontaneouslybeginstobounce(asif

itwerealive)becauseeventhoughsufficientthermalenergyexistsinthefloor

below,thatenergyisunavailableanddispersedinrandomjigglingoftheatoms

ofthefloor.TheSecondLawofThermodynamicsgovernssuchphenomenaand

statesthatentropyintheuniverseneverdecreases.Theentropyincrease(or

energydispersal)conservesenergybutruinsitsquality.High-qualityenergyis

notdistributedbutconcentrated,suchasinabarrelofoil,thenucleusofan

atom,orinphotons(particlesoflight)possessinghighfrequencyandshort

wavelength.Suchphotonsincludeultravioletandvisibleonesthatcausesunburn

andthatpowerplantlife,respectively.Inphysics,suchhigh-qualityenergyhas

lowentropy.

ThephysicistwhomostprominentlylinkedentropytolifewastheNobel

LaureateErwinSchrödinger(1887–1961).InWhatisLife?(1944),Schrödinger

commentedthatanorganism‘tendstoapproachthedangerousstateofmaximum

entropy,whichisdeath.Itcanonlykeepalooffromit,i.e.alive,bycontinually

drawingfromitsenvironmentnegativeentropy…Indeed,inthecaseofhigher

animalsweknowthekindoforderlinesstheyfeeduponwellenough,viz.the

extremelywell-orderedstateofmatterinmoreorlesscomplicatedorganic

compounds,whichservethemasfoodstuffs.Afterutilizingittheyreturnitina

verymuchdegradedform.’Regrettably,Schrödingerintroducedtheconceptof

‘negativeentropy’,whichdoesnotexistinscience,todescribetheordered

structureoffood.Also,inthegrowthofsomeorganisms,theincreaseinentropy

primarilycomesfromheatgenerationratherthanthedegradedformof

metabolicwasteproductscomparedtofood.LinusPauling(1901–94),whowas

arguablythegreatestchemistofthe20thcentury,bluntlyremarkedthat

Schrödinger‘didnotmakeanycontributionwhatever[toourunderstandingof

life]…perhaps,byhisdiscussionof“negativeentropy”inrelationtolife,he

madeanegativecontribution’.

Nonethelessacuriousby-productoftheever-increasingentropyintheuniverse

isthatordered,low-entropystructures,suchasorganisms,springintoexistence.

Infact,efficientproductionofentropyisbestachievedbyso-calleddissipative

structuresinvolvingacoherentstructureofanimmensenumberofmolecules

thatdissipatesenergy.Asimpleexampleisaconvectioncellinboilingwater.

Warmwaterrisesandisbalancedbysinkingwateronitsperiphery.This

circulatingcellhelpstodisperseenergyandsoincreasesentropymore

efficientlythanifthecellwereabsent.Alllivingorganismsarecomplicated

dissipativestructures.However,attemptstodefinelifewiththermodynamics

havesofarfailedtodistinguishclearlybetweenthelivingandnon-living.For

example,thewriterEricSchneiderdefineslifeasa‘farfromequilibrium

dissipativestructurethatmaintainsitslocalleveloforganizationattheexpense

ofproducingenvironmentalentropy’.Afirealsofitsthisdefinition.

Pauling’scriticismaside,Schrödingerarguedcorrectlythatorganismsmustruna

sortofcomputerprogram,whichiswhatwenowcallthegenome.Indeed,life

anywhereprobablyhastopossessagenome.Byagenome,wemeanaheritable

blueprintsubjecttosmallcopyingerrors,whichallowsanorganismtohave

evolvedfromanancestorandprovidesarecipeforlife’sothercharacteristics

suchasmetabolism.Evolution—thechangesinpopulationsoversuccessive

generationscausedbyselectionofindividuals’characteristics—isimportant

becauseitistheonlyprocessthatcanexplainthediversityoflifeandhowthe

featuresoflifethatwerelistedpreviouslywereconfigured.InDarwin’snatural

selectionmechanism,thegeneticvariationinpopulationsofindividualsmeans

thatsomearebetteradaptedforgreaterreproductivesuccessthanothers.Natural

selectionfavoursgenesthatleavemoredescendants,sothatlineagesaccumulate

geneticadaptations.

Mindfulofthecentralityofevolution,astrobiologistsoftendefinelifeas‘aself-

sustainingchemicalsystemcapableofDarwinianevolution’.Unfortunately,this

definitionisnothelpfulifwewanttodesignanexperimenttofindlife.Dowe

havetowaitforevolutiontohappenforapositivedetection?Abetterdefinition

usesthepasttense:‘lifeisaself-sustaining,genome-containingchemicalsystem

thathasdevelopeditscharacteristicsthroughevolution’.Sofar,space-bornelife

detectionexperimentshavenottriedtomeasureageneticmake-up.For

example,NASA’sVikingLanderprobes,whichlookedforlifeonMarsinthe

1970s,weredesignedtorecognizetheEarth-likemetabolismofmicrobesinthe

soil(Chapter6).

ThephilosopherCarolClelandandscientistChristopherChybahavesuggested

thatattemptstodefinelifearelikethoseof17th-centuryscientiststryingto

definewater.Atthattime,waterwasconsideredacolourlessandodourless

liquidthatboilsandfreezesatcertaintemperatures.Withoutatomictheory,no

oneknewthatwaterisacollectionofmolecules,eachconsistingoftwoatomsof

hydrogenjoinedtoanoxygenatom.Byanalogy,perhapswelackthetheoryof

livingsystemsneededtodefinelife.

Manyoftheproblemsindefininglifeboildowntothefactthatwehaveonly

oneexample—lifeonEarth.AllEarth-basedorganismsusenucleicacidsfor

hereditaryinformation,proteinstocontrolbiochemicalreactionrates,and

identicalphosphorus-containingmoleculestostoreenergy.It’sthesamebasic

biochemistryinabacteriumorabluewhale.Soitisdifficulttodistinguish

whichpropertiesoflifeonEarthareuniqueandwhichareneededgenerallyto

qualifyas‘life’.Astrobiologycouldhelpsolvethisconundrumifwefoundlife

beyondEarth.

Thebarenecessitiesoflife

Whilethere’snoperfectdefinitionoflife,therearereasonablegroundstothink

thatcertainatomscommoninterrestrialbiochemistryarelikelytobeusedby

extraterrestrialorganismsandmighthelpusrecognizelifeelsewhere.OnEarth,

thechiefstructuralelementsinbiologyarecarbon,nitrogen,andhydrogen,

whilechemicalinteractionstakeplaceinliquidwater.Inastrobiology,there’s

wideagreementthatlifeelsewhereislikelytobecarbonbasedandthataplanet

withliquidwaterwould,atleast,favour‘lifeasweknowit.’Thesedeductions

arisefromrealizingthatlifeisconstructedfromalimitedtoolkit,theperiodic

tableofchemicalelements,whichisthesamethroughouttheuniverse.

Infact,carbonistheonlyelementcapableofforminglongcompoundsof

billionsofatomssuchasDNA(deoxyribonucleicacid).Consequently,only

carbon-basedextraterrestriallifeseemsabletohaveagenomeofcomparable

complexitytoterrestriallife.Carbonalsohasavarietyofotherpropertiesthat

allowauniquechemistryofitscompounds,sufficienttospawnthedisciplineof

organicchemistry.Carbon’sspecialpropertiesincludetheabilitytoformsingle,

double,andtriplebondswithitselfaswellasbondswithmanyotherelements.

Carboncanalsobuildthree-dimensionalcomplexitybyforminghexagonalrings

thatjointogether.

Becauselifehastogetstartedandpropagate,it’sprobablethatthemainatomsof

lifeareabundantones.Carbonisfourthincosmicabundanceafterhydrogen,

helium,andoxygen.Indeed,astronomershavefoundthatmanynon-biological

organicmoleculesalreadyexistinspace.Thesefreebiesmightserveas

precursorstolifegettingstarted(seeChapter3).Forexample,around30per

centbymassofthedustbetweenthestarsisorganicmaterial.So-called

carbonaceouschondritemeteoritesandinterplanetarydustparticlesinourown

SolarSystemcontainupto2percentand35percentorganiccarbonbymass,

respectively.

Becausesiliconhaschemicalpropertiessimilartocarbon,itissometimes

assertedthatsiliconmightallowanalternativeextraterrestrialbiochemistryto

carbon-basedmolecules,despitebeingabouttentimeslesscosmicallyabundant

thancarbon.Butinwater,atleast,siliconcompoundstendtobeunstableand

siliconeasilygetslockedintosolidsiliconoxides.Carbondioxideisagasat

commonplanetarytemperaturesanddissolvesinwatertoconcentrations

sufficientfororganismstousecarbondioxideasacarbonsource.Silicon

dioxide,incontrast,isaninsolublesolid,suchasquartz.Silicon’sbondswith

oxygenandhydrogenarestrong,whereascarbon–oxygenandcarbon–hydrogen

bondsaresimilarinstrengthtothecarbon–carbonbond,whichallowscarbon-

basedcompoundstoundergoreactionsofexchangeandmodification.Silicon–

hydrogenbondsalsotendtobeeasilyattackedinwater.Thestabilityofsilicon-

basedmoleculesrequireslowtemperaturestoslowdownreactionsthatwould

otherwisedestroythem.Appropriatelycoldsolventsincludeoceansofliquid

nitrogenonicyplanetsfarfromtheirstars.Atpresent,suchsilicon-basedlife

remainspurelyspeculative.

However,astablemediumisnecessaryforbiochemicalprocessessuchas

metabolismorgeneticreplication;onEarththismediumisliquidwater.For

extraterrestriallife,themediumcouldbeanotherliquidoradensegasaslongas

itdoesn’teasilybecomeasolidintheprevailingenvironment.Nonetheless,

water(H

O)hassomeuniqueproperties.Unlikeitssmellytwin,hydrogen

sulphide(H

S),whichcondensestoanastyliquidonlyat–61°C,waterturnsto

liquidbelow100°Catnormalpressures.Liquidwater’sstabilityoccursbecause

theoxygenatominwatermoleculesisslightlynegativelychargedandallowsa

relativelystrong‘hydrogenbond’totheslightlypositivehydrogenatomsof

otherwatermolecules.Sulphurprovidesweakerhydrogenbondsbetween

hydrogensulphidemolecules.Wateralsoformsstrongerhydrogenbondsto

otherwatermoleculesthantomoleculesofoilysubstances.Asaconsequence,

oilsseparatefromwater,whichallowscellmembranestoformandprovide

homesforgenesandmetabolicprocesses.

Anotherunusualpropertyofwateristhaticeislessdensethanliquidwater.

Whenwaterfreezes,themoleculesalignintoring-likestructurescontaining

openholesontheatomicscale.Ificeweredenserthanliquid,thecoldbottomof

lakesandseaswouldcollectice,whichwouldbeinsulatedandremainfrozen.

Seaswouldfreezefromthebottomupandbecomeuninhabitable.Thiswould

arisebecausesunlightwouldbereflectedbackintheareaswheretheicereached

thesurface,causingcoolingandmoreicetoaccumulate.Gradualfreezingmight

betheunfortunatefateofseasofotherliquidssuchasammonia.Ammoniaisa

liquidfromabout–78°Ctoabout–33°Catapressureofoneatmosphere.But

anyseasofammoniawouldtendtosolidifyfromthebottomup,unlikeseasof

waterthatremainliquidevenifcoldtemperaturescauseanicecover.

OnEarth,wefindmicrobeswhereverthere’sliquidwater(excludingsterilized

apparatus),so‘lifeasweknowit’isasmuch‘waterbased’as‘carbonbased.’

Consequently,forSolarSystemexploration,thedetectionofliquidwaterorits

pastpresenceprovidesanobjectiveforplanetaryprobes,suchasthosevisiting

Mars.Nonetheless,itispossibletoconceiveoforganicsolventsasalternatives

towater,whichispotentiallyimportantforTitan,Saturn’slargestmoon(Chapter

6).

Anotherobservationfromterrestriallifeisthatjustsixnon-metallicelements—

carbon,hydrogen,nitrogen,oxygen,phosphorus,andsulphur—makeup99per

centoflivingmaterialbymass.Theseelementsareoftenabbreviatedas

‘CHNOPS’,butIprefer‘SPONCH’,whichiseasiertosay.WefindHandOin

water,whichmakesupmostlivingtissue,andC,H,andOinthenucleicacidsof

geneticmaterialandincarbohydrates.C,H,N,andSexistinproteins,whilePis

essentialfornucleicacidsandenergy-storagemolecules.Consequently,

detectingtheSPONCHelementsinchemicalformsthatlifecoulduseisanother

practicalgoalforplanetaryspaceprobessearchingforEarth-likelife.

Thesignificanceoflifeelsewhere

Youmightwonderwhetherthediscoveryofsimple,microbial-like

extraterrestrialswouldreallymatter.Butifwecouldfindasingleinstanceoflife

thatoriginatedelsewhere,itwouldprovethatlifeisnotamiracleconfinedto

Earth.Wewouldn’tbealone.EventhesimplestmicrobesnativetoMarsorto

Europa’soceanswouldchangethebalanceofprobabilitiesthatlifeexists

elsewhereinthegalaxyfortheywoulddemonstratethatlifecanoriginatetwice

withinonesolarsystem.Atthemoment,wehavenoconvincingevidenceoflife

beyondEarth.However,inChapter6,Iwillarguethatatleastnineotherbodies

inourSolarSystemmightbehabitabletoday,ifwekeepanopenmind.Solar

Systemastrobiologyisfarfromsettled.

Asecondsignificantfactorinfindinglifeharksbacktothedifficultiesof

defininglife.TheplanetaryscientistCarlSagan(1934–96)commentedinhis

bookCosmicConnection(1973)that‘thesciencethathasbyfarthemosttogain

fromplanetaryexplorationisbiology’.Anexaminationofextraterrestriallife

wouldbeofprofoundsignificancenotonlyinidentifyingthoseelusive

characteristicscommontoalllifebutalsoinsheddinglightonhowlife

originated,whichremainsunsolved.

Chapter2

Fromstardusttoplanets,theabodesforlife

Tomakeanapplepiefromscratch,youmustfirstinventtheuniverse.

CarlSagan(1980)

Whentheuniversebegan,temperatureseverywherewerefartoohotforatomsto

bestable,letalonejoinupintocomplexbiologicalmolecules.Lifeexists

becausefollowingaBigBang13.8billionyearsago,ahot,densecosmos

expandedandcooled.Asitdidso,atoms,galaxies,stars,planets,andlifearose.

Here,weexaminehowthisprocessproducedanabodeforlife—theEarth.

Westartwiththestructureofthepresentuniverse,whichprovidestheclues

aboutitshistory.Imagineajourneythatgoestotheedgeoftheobservable

universe.Atthespeedoflight—300,000kmpersecond—itwouldtakeonly1.3

secondstogettotheMoonatitsdistanceof384,000km.AdiagramoftheEarth

representedasaspotof2.7mmdiameterandtheMoonlooksasfollows:

Thisscaleisfairlyeasytograsp.Butitchallengesourimaginationwhenwe

realizethattheSunonthesamescalewouldbe30cmindiameterandwewould

havetoplaceitjustover30mawayfromthisbook.Theneareststar,Proxima

Centauri,whichissmallcomparedtotheSun,wouldbeabout4cmindiameter.

Tokeeptoscale,wewouldhavetoplaceit8,650kmaway,whichisroughlythe

flightpathfromSanFranciscotoLondon.

Continuingourvoyageatthespeedoflight,itwouldtake8.3minutestofly

fromtheEarthtotheSunandalittleoverfourhourstothentraveltotheaverage

orbitaldistanceofNeptune,theoutermostoftheeightplanets.After4.2years,

wewouldreachProximaCentauri.Withthehugedistancesinvolved,wedefine

thedistancetravelledinayearatthespeedoflight,some9,500billionkm,asa

lightyear,sothatProximaCentauriis4.2lightyearsaway.

TheSunandProximaCentauriaretwoofabout300billionstarsintheMilky

Waygalaxy,whichisadiscsome100,000lightyearsacrosswithstars

concentratedinspiralarms.Galaxiescontainmillionstotrillionsofstars,sothe

MilkyWayismoderatelylarge.TheSolarSystemsitsintheOrionArm,two-

thirdsoutfromtheGalacticCentre.Thisarmappearsunremarkablecomparedto

someothersthataremorerichlypopulatedwithstars.Butit’spossiblethatthe

SolarSystem’slocationinthegalacticstickswasactuallyvitalforterrestriallife.

TheEarthmayhaveavoidedcertaincatastrophes,suchasproximityto

explodingstars.Ifso,theremaybeaparticularregioningalaxiesfavourablefor

life,calledthe‘GalacticHabitableZone’,discussedinChapter7.

Onalargerscale,therearemorethan100billiongalaxiesintheobservable

universearrangedintogroupsandsuperclusters.Withinadiameterofabout10

millionlightyearscentredbetweentheMilkyWayandournearestspiralgalaxy,

theAndromedagalaxysome2.5millionlightyearsaway,thereareroughlyfifty

galaxies,formingtheLocalGroup.Inturn,thisgroupisoneofahundredorso

withinasphereof110millionlightyears’diameter,comprisingtheVirgo

Supercluster.Mapsthatcoverbillionsoflightyearsshowfilamentsoftiny

scatteredpointswhereeachpointisagalaxy.Inthreedimensions,galactic

filaments,whicharethelargeststructuresknowntohumankind,joinupintoa

webseparatedbyvastvoids.Thewholefantasticstructurelooksasifitwere

spunbyacrazyintergalacticspider.

Howbigistheobservableuniverse?Ifspacehadnotexpanded,thefarthest

distancewouldbe13.8billionlightyears,whichisthattraversedbyaphoton—a

particleoflight—sincetheBigBanghappened13.8billionyearsago.Butspace

hasexpanded.Sotheactualsizeoftheobservableuniverseisnowabout47

billionlightyearsacross.Suchvastnessisaconsiderationforastrobiology

becausesurveysofplanetsaroundstarsjustwithintheMilkyWaysuggestthat

eachstarhostsatleastoneplanetonaverage.Somefractionofplanetsoughtbe

habitable,perhapsatleast1percent,sothenumberofpotentialabodesforlife

mightexceedatrillionbillion.

ThestructureoftheuniversetracesbacktotheBigBang.Inthe1920s,

telescopicobservationsbytheAmericanastronomerEdwinHubbleshowedthat

galaxiesaremovingawayfromeachotheronthelargescaleasspaceexpands

betweenthem.Thus,goingbackfarintime,everythingmusthavebeen

scrunchedupandveryhot.Thisconsiderationledtotherecognitionofstrong

evidencefortheBigBang.Itsafterglow,theCosmicMicrowaveBackground,

permeatestheentireuniverse.IftheBigBangistrue,physicsdictatesthatbefore

theearlyuniversecooled,itmusthavebeenanopaquefireballmadeofelectrons

andprotons(theelectricallynegativeandpositiveelementaryparticlesthatmake

upatoms),photons(particlesoflight),andsomesmallgroupsoffusedprotons.

Some380,000yearsaftertheBigBang,itbecamecoolenoughthatelectrons

wereabletojoinwithprotons,orgroupsofprotons,toformthetwosmallest

atoms,hydrogenandhelium.Atthatpoint,theuniversebecametransparentto

lightbecausepreviouslyphotonshadbeenscatteredbythefree-floating

electrons.Thispriorsituationwasanalogoustothewaythatlightbounces

aroundofftinydropletsinafogsothatyoucan’tseethrough.Sincelosingits

opaqueness,theuniversehasexpandedbyaboutafactorof1,000,andthe

wavelengthoftherelicphotonsfromtheBigBanghasbeenstretchedthesame

amount,changingthephotonsfromredlighttomicrowave.Amazingly,when

youtuneanoldanaloguetelevisionorradiobetweenchannels,theleftover

radiationfromtheBigBangcontributestothestatichiss,albeitatalevelof

about1percentorless.Infact,in1964,thisnoiseinalargeradiohornwashow

themicrowavebackgroundwasdiscovered.Pigeondroppingsinthehornwere

blamedinitially;butaftercleaningupandshootingtheunfortunatepigeons,the

realculpritwasidentifiedasthebeginningoftheuniverse.

GalaxiesappearedafewhundredmillionyearsaftertheBigBang.Someplaces

hadveryslightlymorematerialthanaverageandsohadhighergravity.

Clumpingproducedgalaxies,andwithinthegalaxies,onasmallerscale,gas

cloudscollapsedundertheirowngravity.Theinteriorofeachshrinkingcloud

heatedupasgasparticlescollided,eventuallymakingahot,glowingballofgas

—astar.

Partofthe‘astro’inastrobiologycomesfromthefactthatalloftheatomsused

bylifeexceptforhydrogenwerecreatedinsidestars.Thefirststarswouldhave

beenmadeonlyoftheelementssynthesizedintheBigBang:hydrogen,

comprisingthree-quartersofthemass,andhelium,whichmadeuptherest

exceptforatraceoflithium.Theoxygeninwater,thenitrogeninproteins,orthe

carbonineveryorganicmolecule—noneoftheseelementswaspresentatfirst.

Buteventuallystarsmadethem.

Tounderstandhowstarsmakeelements,considerhowtheSunshines.Insidethe

Sun,immenseheatstripseachatomdownintoitsconstituents:apositively

chargednucleusandnegativelychargedelectrons.Thetemperatureatthecentre

oftheSun,some16milliondegreesCelsius,isenoughtofusethenucleioffour

hydrogenatomsintoaheliumnucleus,whichisanuclearreactionthatreleases

photons.Eachphotonthenenduresaone-million-yearjourneyfromtheinterior

oftheSuntospace.Ittakessolongbecauseeachphotoniscontinuallyabsorbed

andemittedasitencountersmaterial.Thephotonalsolosesenergy.Itstartsout

asahigh-energygammarayandonaverageturnsintoalower-energyphotonof

visiblelightbythetimeitescapesfromtheSun.Inthe1950s,physicists

reproducedthekindofnuclearreactionsthatoccurinsidetheSunwithhydrogen

bombs.CoresofstarslikeourSunareakintohydrogenbombsthat,ineffect,

can’texplodebecausetheyarecontainedbytheweightofmaterialabovethem.

TheSunwillnotfusehydrogeninitscoreforeverandtherepercussionswill

destroylifeonEarth(partlyansweringastrobiology’squestionof‘What’sthe

futureoflife?’).Inmoststellarcores,theaccumulationofhelium‘ash’causes

temperaturestodroptoolowtosupportfurtherhydrogenfusion.Atthispoint,

thestarshrinksunderitsownweight,whichcausesthetemperaturetoriseuntil

itigniteshydrogenfusioninashellsurroundingthecore.Theenergyrelease

causesouterlayersofthestartoexpand,cool,andredden.Thisishowared

giantforms,suchasAldebaran,thebrighteststarintheconstellationofTaurus,

theBull.TheSunwilleventuallybecomearedgiantandswelltwo-hundredfold

by7.5billionyears’time,probablyengulfingtheEarth.Theweightoffurther

heliumasheventuallysqueezesaredgiant’scoretoatemperatureof100–200

milliondegreesCelsius,whichisenoughtofuseheliumnucleiandmakecarbon

andoxygen.Inturn,ahelium-burningshellcaneventuallysurroundacoreof

carbonandoxygen‘ash’.StarswithfourtoeighttimesthemassoftheSuneven

endupfusingthecarbonandoxygenintoheavierelements,includingneonand

magnesium.

Generally,thedeaththroesofaSun-likestarinvolvesheddingouterlayersinto

space.Theseshellsofglowinggasarecalledplanetarynebulaebecausethey

looksomewhatlikeplanetsthroughlow-magnificationtelescopes,buttheyhave

nothingtodowithplanets.Theremnantsofthestarcooldownintoawhite

dwarfwitharadiuscomparabletothatoftheEarthbutanenormousdensity.In

theory,awhitedwarfstopsshiningaftertenstohundredsofbillionsofyears,

producingablackdwarf.Buttheuniverseisnotyetthatold.

StarslargerthanabouteighttimesthemassoftheSuneventuallyexplodeas

supernovae.TheSunisabouthalfwaythroughitsten-billion-yearphaseof

hydrogenfusioninitscore,butthesemassivestarsspendlessthan60million

yearsinthesamephasebeforebecomingredsupergiants,ofwhichBetelgeuse,

intheconstellationofOrion,isanexample.Insuchstars,nuclearfusionin

shellsaroundthecoreproducestheelementsneon,magnesium,silicon,andiron.

Ironisgenerallytheheaviestelementmade,althoughsomeheavierelementsare

alsoproducedwhenfreeneutronsareaddedtoexistingnuclei.(Neutronsare

particlesthathavenoelectricalchargeandarecommonlyfoundinatomic

nuclei.)Whenthefuelinsuchastarrunsout,thecoreissocompressedthat

negativelychargedelectronsamalgamatewiththepositivelychargedprotonsof

theironnuclei.Theelectricalchargescanceloutandunchargedneutronparticles

arecreated.Asaresult,thestellarcoreshrivelstoabout12kmdiameter,

formingsomethinglikeagiganticatomicnucleusmadeonlyofneutrons.

Becauseoftheshrinkage,therestofthestarcollapsesontothedenseneutron

core,andaviolentreboundcreatestheincrediblybrightsupernova.

Elementsheavierthanironaregeneratedanddistributedbysupernovae.Afew

secondsafterasupernova,theouterlayersofthestarareheatedtoanincredible

10billiondegreesCelsiuswhilethebreak-upofnucleifromdeeperlayers

suppliesabundantneutronstofuelreactionsthatmakeheavyelements.This

cosmicalchemycreatestheheavy,preciouselementssuchasgold,silverand

platinum.Importantly,thematerialblownoutinasupernovaformsthebasisof

newgenerationsofstarsandabodesforlife,theirplanets.Inthecaseofstars

reachingtensofsolarmasses,althoughasupernovastilloccurs,thecollapseat

thecentreproducesablackhole—anobjectsomassivethatnothingescapesits

gravity,includinglight.

1.TheHertzsprung–RusselldiagramfortheSunandnearbystars.

TemperaturesareinKelvin,whichis273plusthetemperatureindegrees

Celsius;theKelvinscaleisdefinedsothat0Kor‘absolutezero’iswhereall

molecularmovementstops

Thephasewhenastarconvertshydrogentoheliuminitscoreisthemain

sequencelifetime,whichisgenerallytheintervalweconsideroptimalforlifeto

thriveonplanetsaroundthestar.The‘mainsequence’referstoadiagonal

swathefromupperlefttolowerrightonthemostfamousgraphinastronomy,

theHertzsprung–Russell(H–R)diagram(Fig.1),namedafteritstwo

originators.Thegraphplotsastar’sluminosityversusitssurfacetemperature.

Bya‘surface’,astronomersdon’tmeanahardsurfacebutthelevelinastar’s

atmospherewheremostlightemerges,whichisasdeepaswecansee.

Oddly,thetemperatureaxisrunsbackwardsfromhightolowintheH–R

diagram.Thepurposeistomatchthecolourcodingofstarsfromhotblue-white

starstocoolerredoneswiththelettersO,B,A,F,G,K,andM.Thelettergives

astar’sspectralclass.Generationsofastronomystudentshaveremembered

spectraltypeswiththemnemonic‘OhBeAFineGirl/GuyKissMe!’(theletters

don’tstandforanythingandhaveoriginsin19th-centuryastronomy,whichneed

notconcernus).

Onthemainsequence,themostmassivestarsplotattheupperleftandthe

lightestatthelowerright.Whateveritsmass,amainsequencestariscalleda

dwarf—suchastheSun,aG-typedwarf.Thelifetimeonthemainsequencecan

beover50billionyearsforcoolreddwarfs.

Theimportancetoastrobiologyofthewaystars‘live’and‘die’iswideranging.

OurSunisamiddle-aged,mainsequencestarwithstablesunlightthatfuels

mostlifeonEarththroughphotosynthesis.Asmentionedattheoutset,theatoms

oflifeweregeneratedinredgiantsandsupergiants.Also,oxygen,silicon,

magnesium,andironaremadefromintegralnumbersofheliumnucleiandso

areparticularlyabundantproductsofnuclearfusion,whichissignificantbecause

theseatomsaretheonesthatmakerocks.Rockyplanets,liketheoneweinhabit,

areanaturalconsequenceofthephysicsofstarlight.WealsoknowthattheSun

isatleastasecond-generationstarbecausewehavesupernovaelementsonEarth

suchasgold.ButsincetheSunisonly4.6billionyearsoldina13.2billion-

year-oldMilkyWay,manystarscameandwentbeforelifearoseonEarth.Did

earlierstarssupportplanetsandlife,orevenintelligentlife,andwhathappened

tothem?Thisleadsustothequestionofhowourownplanetformed.

Gettingaplacetolive:whereplanetscomefrom

IdeasfortheoriginoftheSolarSystemhavealongheritage.In1755,Immanuel

KantsuggestedthattheSolarSystemcoalescedoutofadiffusecloudinspace.

Later,in1796,themathematicianPierreSimon,theMarquisdeLaplace,

elaboratedthenotion.ThebasicKant–Laplaceconceptisknownasthe‘nebular

hypothesis,’afternebula,theGreekwordforcloud.

Thehypothesisstartswiththeideathatsomepartofthecloudisslightlydenser

thanothersandattractsmaterialbygravity.Itislikelythatthecloudalsohas

someslightinitialrotation,sothattheshrinkingcloudspinsfaster,likeanice

skaterdrawinginherarms.Therandommotionofthegasanddustwilloppose

materialattractedalongtheaxisofrotationbutmatterconvergingintheplaneof

rotationwillalsoberesistedbythespin.Asaresult,thecloudflattensintoa

disc.TheSunformsinthecentre,whileplanetscoalesceintheplaneofthedisc

fromsparsematerial.Thisisconsistentwiththemassoftheplanetsbeingonly

0.1percentoftheSun’smass.Becausetheyformfromadisc,planetswillallbe

inthesameplaneandtheywillallorbittheSuninthesamedirection,aswe

observe.

Inrecentdecades,itwasthoughtthatevidencefromisotopessuggestedthata

shockwavefromanearbysupernovatriggeredthecollapseofthenebula.

Isotopesareatomsthatcontainthesamenumberofprotonsintheirnucleusbuta

differentnumberofneutrons.TheGreekroot,isostopos,means‘equalplace’,

whichreferstothesamelocationintheperiodictableofelements.Sometimesan

isotopehasanucleusthatistoobigtobestable,andbreaksapartbyradioactive

decay.Anunstablealuminium-26atom(whichhasanucleusof26particles=13

protons+13neutrons)decaysintostablemagnesium-26(containing12protons

+14neutrons).Inanysampleofaluminium-26atoms,thetimeittakesforhalf

ofthemtochangeintomagnesium-26,thehalf-life,is700,000years.The

presenceofmagnesium-26insomemeteorites,whichmusthavebeenproduced

relativelyquicklyfromaluminium-26,wasthoughttosuggestanearby

supernovawhentheSolarSystemformedbecausemassivestarsmake

aluminium-26andtheirsupernovaedistributeit.

However,in2012,newmeasurementsinmeteoritesshowedthatthelevelsof

iron-60,whichisanisotopeonlyformedinsupernovae,aretoolowforanearby

supernova.Toexplainabundantaluminum-26,aneighbouringmassivestar

(perhapsmorethantwentytimesthesolarmass)ofatypeknownasaWolf–

Rayetstarmayhavesheditsouterlayersandspreadaluminum-26intothesolar

nebula.Aluminum-26wasadominantheatsourceinthesolarnebulaforthefirst

fewmillionyears.Bymeltingiceintheearliestrockymaterial,thealuminum-26

causedwatertogointohydratedmineralswhereitwassafe,unlikeicethat

evaporates.Ifthealuminum-26hadnotbeenpresent,theEarthmightnothave

gainedwater-richmineralsanditmighthaveneitheroceansnorlife.

Thenebularhypothesisexplainsthebroaddistributionofdifferenttypesof

planet.TheplanetsintheinnerSolarSystem(Mercury,Venus,Earth,andMars)

arerelativelysmallandrocky,whilethoseintheouterSolarSystem(Jupiter,

Saturn,Uranus,andNeptune)aregiants.Whenthediscwasforming,thematter

drawninwardsgainedenergyofmotionandthecentreofthedisc,wheretheSun

formed,becameextremelyhot.Thiscreatedatemperaturegradientfromthehot

centreofthedisctoacoldexterior.OutsidetheSun,pressureswerelow,which

meantthatsubstancesexistedeitherassolidsorgasesbutnotliquids.Inthe

innerdisc,itwasfartoohotforgasessuchaswatervapourtoformice,but

metalandrockvapourcouldcondensenearlyanywhereinthedisc.

Consequently,planetsthatformedintheinnerregion,suchastheEarth,ended

upwithiron-richcoressurroundedbyrockymantles.Incontrast,water

condensedintoicefromjustinsidetheorbitofJupiter(‘theiceline’)andfarther

out,providingmorematerialtomakelargerplanets.Methanecouldalso

condenseasanicefromtheorbitofNeptuneoutwards.

Thegiantplanetsareconsideredtohaveformedbeforetherockyones.Jupiter

andSaturnprobablyformedwhenrockycoresreachedasizeofabouttenorso

Earthmassesthathadsufficientgravitytoattractmoreandmorehydrogenand

heliumgasdirectlyfromthediscuntilallthatwasavailableintheirorbitwas

suckedup.Becausetheyarehugeballsofmostlygas,wecalltheseplanetsgas

giants.Thisprocesshappenedwithinabout10millionyearsaftertheformation

oftheSun.UranusandNeptunearesmaller,anddrewinagreaterproportionof

icysolids,sowecalltheseplanetsicegiants.UnliketherapidgrowthofJupiter,

theformationoftheinnerrockyplanetswasspreadover100–200millionyears.

Planetesimals,whichare‘piecesofplanet’,coalescedtomakelarger,rocky

objectscalledplanetaryembryoswithasizebetweenthatoftheMoonandMars.

Inturn,severalplanetaryembryosmergedintoVenusandEarthandfewerinto

MercuryandMars.

Althoughtheinnerplanetsaccumulatedmaterialintheirlocality,gravitational

nudgesfromthegiantplanets,particularlyJupiter,wouldhavesent

planetesimalscareeringintotheinnerSolarSystem.Scatteredhydratedasteroids

thatoriginatedfrombeyondtheorbitofMarswerelikelyresponsiblefor

bringingwatertotheEarththateventuallyturnedintoouroceansandlakes.We

drinkasteroidwater.ComputersimulationsshowthatJupiterejectedmore

water-richmaterialthanitscatteredinward,sothatifJupiterhadhadaless

circularorbitandejectedevenmorewater-richmaterial,Earthmighthaveended

upwithoutoceansandlife.

Wealsoknowthatplanetsdonotnecessarilystayputintheirorbits.So-called

hotJupitersareexoplanetssimilarinmasstoJupiterthatorbitatleasttwiceas

closetotheirhoststarsastheEarthorbitstheSun.Suchexoplanetscannothave

formedwheretheycurrentlyresidebecauseitwouldhavebeentoohot.Itturns

outthatplanetsmigratebecauseoflargetailsofgasanddustinthenebulathat

accompanytheirformationaswellasthegravitationalinfluencefromother

bodies.Sothetraditionalnebulahypothesisisnuancedbyplanetarymigration

thatcoulddestroyorfavourplanetaryhabitabilityinextrasolarsystems,

dependingonthedetails.Inthenextchapter,I’lldiscusstheideathateventhe

giantplanetsofourownSolarSystemmayhavemigratedsomewhat.

Nonetheless,theessentialideaofthenebularhypothesis—thattheplanets

formedfromadisc—wasconfirmedinthe1980swhendiscsofdebrisaround

youngstarswereseenthroughtelescopes.Infact,thereisstillsomeleftover

debrisinourownSolarSystem.Cometsareicybodiesandasteroidsarerocky

rubblethatwereneverassimilated.Occasionally,smallchunksthathavebeen

knockedoffasteroidsthroughcollisionsendupfallingontotheEarth’ssurface

asmeteorites.Consequently,meteoritesprovideuswithkeydataabouttheearly

SolarSystem.

TheageoftheEarthandtheMoon

Perhapsthemostprofoundinformationgleanedfrommeteoritesistheageofthe

EarthandSolarSystem.Eversincethe18thcentury,thevastlayersof

sedimentaryrockseenbygeologistshadledthemtosuspectthattheEarth,and

hencetheSolarSystem,mustbeofgreatage,butproofwaslacking.

ThefirstpersontoattempttomeasuretheageoftheEarthwasanEnglish

geologist,ArthurHolmes,whohadtheideaofexaminingleadisotopesformed

fromradioactiveuranium.Uranium-238decaysintoacascadeoffurther

unstableisotopesofotherelementsuntilreachingastableleadisotope,lead-206.

Thehalf-lifeforuranium-238tochangeintolead-206is4.47billionyears.

Anotherradioactiveisotope,uranium-235,decaysintolead-207withahalf-life

of704millionyears.So,bymeasuringtheamountsoflead-207andlead-206in

differentmineralgrainsofarock,youcandeterminetheageoftherock,because

althoughtheremayhavebeendifferentamountsofuraniumineachgrain,the

fixeddecayratesaddthesameratiooflead-206tolead-207inagiventime.In

1947,HolmesappliedhismethodtoapieceofleadorefromGreenland,and

estimatedthattheEarthformedby3.4Ga(whereGameans‘Gigaanna’,

equivalentto1,000millionyearsor‘billionsofyearsago’).

Holmeshadtwoproblems.First,hecouldneverbesurethateventheoldestrock

hecouldfindwasasoldastheEarthitself.Second,forapreciseage,Holmes

neededtoknowthesmall,originalratioofleadisotopespresentwhentheEarth

formed,so-called‘primevallead’,beforesubsequenturaniumdecayadded

furtherleadatoms.ClairPatterson,anAmericangeochemist,realizedthatthe

firstproblemcouldbeavoidedbylookingatmeteoritesbecausetheseare

leftoverbuildingmaterialsthatformedaroundthesametimeastheEarth.He

alsorecognizedthatcertaintypesofmeteorite,theironmeteorites,contain

negligibleuraniumandsotheirratioofleadisotopesprovidesameasureof

primevallead.Withthisapproach,in1953,PattersonaccuratelyagedtheEarth

at4.5Ga.Hewassoexcitedthathefearedaheartattack,andhismotherhadto

takehimtohospital.

Sincethen,improvedtechniquesusingradioactiveisotopeshavegivenusa

timelinefortheeventssurroundingtheEarth’sformation.Theveryoldestgrains

inmeteoritessuggestthattheSolarSystemformedat4.57Ga.TheEarthformed

slightlylaterat4.54Ga.Then,around4.5Ga,theEarthwasapparentlyhitbya

Mars-sizedobject,whichisnamedTheiaaftertheGreekgoddesswhogavebirth

totheMoongoddessSelene.Accordingtothisgiantimpacthypothesis,debris

thatwasblastedoutfromtheimpactwentintoorbitaroundtheEarthand

coalescedtoformtheMoon.

TheMoonisimportantforastrobiologybecauseitsgravitystabilizesthetiltof

Earth’saxistotheplaneofitsorbit,whichhelpstheEarthmaintainarelatively

steadyclimate.Currently,theEarth’saxisistilted23.5degrees,butifitwere

abletovarywidely,greatclimaticswingswouldoccur.Forexample,at90

degreestilt,theEarthwouldbetippedonitssideandicewouldformseasonally

attheequator.ComputersimulationsshowthatifwehadnoMoon,theEarth’s

axialtiltwouldvarychaoticallyoverperiodsofmillionsofyearsandhavea

largerange,potentiallyfrom0tomorethan50degrees.Microbiallifecould

probablywithstandlargeclimateswings.Butadvancedanimallifeand

civilizationssuchasourswouldbechallenged.

Infollowingourastronomicaltrail,wehavenowarrivedatthepointofthe

creationofanabodeforlife,ourownplanet.Therewerecontingenciesin

whetherthewaternecessaryforlifewasdeliveredtotheEarthandcontrolson

whereanEarth-likeplanetmightoccur.Then,afterEarthformed,howandwhen

didlifearise?

Chapter3

Originsoflifeandenvironment

TheearlyEarth

SolittleisknownabouttheveryearliestaeonofEarthhistorythatitdoesn’t

evenhaveanofficialname.Informally,itiscalledtheHadean,whichstarted

around4.5GawhentheMoonformed,andendedatadatethathasn’tbeen

agreeduponbutisusuallytakenaseither3.8or4.0Ga.Itisprobablethatlife

originatedintheHadean,butsofarwehaven’tfoundevidencebecausethereare

nosedimentaryrocksfromthistime.Suchrocksconsistoflayersofsediment

grainslaiddowninwaterorfromairfall,andsobestpreservetracesofbiology

ortheenvironment.Consequently,ourideasofwhathappenedduringthe

Hadeanhavetobeconstructedfromsparsedataaidedbythetheoretical

constraints.

TheorysuggeststhatheatfromthegiantimpactthatformedtheMoonwould

haveturnedrockintogas.Theatmosphereofvaporizedrockwouldhavelasted

forafewthousandyearsandthencondensedandraineddownonamolten

magmasurface,whicheventuallysolidifiedintoacrust.Subsequently,the

atmospherewouldhaveconsistedmainlyofextremelydensesteamforafew

millionyearsbeforeitcondensedtoformoceans.

Whendidcontinentsbegintoform?Wegetsomecluesfromtinymineralgrains

lessthan0.5mmacrossthatareleftbehindfromthefirsthalfbillionyearsorso

ofEarthhistory.Thesearezircons,crystalsofzirconiumsilicatewithchemical

formulaZrSiO

,whereZriszirconium,Siissilicon,andOisoxygen.Zircons

aresotoughthattheyremainevenaftertherockinwhichtheywereoncehosted

haserodedaway.Somezirconsasoldas4.4to4.0Gahavebeenfoundin

fossilizedgravelintheJackHills,whichareaboutathousandkilometresnorth

ofPerthinwesternAustralia.Thezirconscontainflecksofquartz,whichisa

crystallineformofsilica,SiO

.Thequartzmayhavebeenderivedfromgranites,

whichisthesilica-richtypeofigneousrockthatmakesupmuchofthe

continents.Inthisway,zirconssuggestthatEarth’scontinentalcrustexistedas

longagoas4.3Ga.Isotopessupportthisinference.Someancientzirconsare

enrichedinstableoxygen-18atomsrelativetostableoxygen-16ones.Such

enrichmentoccurswhensurfacewatersmakeclaysandthemudisthenburied

andmeltsunderground,passingtheisotopicsignaturetoigneousrocks.

IntheHadean,theEarthwasprobablyhitbyafewhugepiecesofdebrisleft

overfromSolarSystemformation,butnoneasbigastheMoon-forming

impactor.Theenergyofasmallnumberofverylargeimpactscouldhave

vaporizedtheentireoceanoritsupperfewhundredmetres.Ifso,lifewould

havehadtorestartoritmighthavebeentrimmedbacktoonlythosemicrobes

shelteredundergroundthatwereabletosurvivetheheat.Infact,partially

sterilizingimpactsmayexplainthenatureofthelastcommonancestorforall

lifeonEarth.Geneticstracesthecommonancestortoathermophile(seeChapter

5)—amicrobethatlivesinhotenvironments.Essentially,DNAanalysisimplies

thatyour‘great-great-great-…grandmother’wasathermophile,ifyouinsert

enough‘greats’.Thismaybebecausethermophilesweretheonlysurvivorsof

hugeimpacts.

TheEarthtodayisnotentirelyfreefrompotentiallycatastrophicimpacts.For

example,Chironisacomet-likeobjectabout230kmacrossintheouterSolar

SystemthatcrossestheorbitofSaturn.Onedaywithinthenext10millionyears

orso,anudgefromSaturn’sgravitywillflingChirontowardstheSunoraway

fromit.Intheformercase,Chiron’schancesofimpactingtheEarthwouldbe

lessthanoneinamillion.ButifChirondidhit,theheatwouldturntheupper

fewhundredmetresoftheoceanintosteam,andlandwouldbesterilizeddown

toaboutfiftymetres’depth.Thermophilesmightbecometheancestorsof

subsequentlifeinasortofevolutionarydéjàvu,iftheimpactexplanationfora

thermophileancestoriscorrect.

ThelasthurrahofthebigHadeanimpactsisknownastheLateHeavy

Bombardment,whichhappenedabout4to3.8Ga.CratersontheMoonbear

witnesstothismassivebombardment.RocksbroughtbackbyApolloastronauts

havebeendatedusingradioisotopesandindicatethatmanycraterswere

producedwithinthesame200-million-yearinterval.

TheleadinghypothesisforexplainingtheLateHeavyBombardmentwas

devisedbyastronomersinNice,France,andisthereforecalledtheNicemodel.

ItreliesontheastonishingideathattheorbitsofJupiterandtheothergiant

planetsshiftedattheendoftheHadean.CalculationssuggestthataftertheSolar

Systemformed,mutualgravitationaleffectscausedSaturnandJupitertoreacha

statecalleda‘resonance’inwhichSaturnorbitedtheSunonceforeverytwo

Jupiterorbits.Regularalignmentcreatedperiodicgravitationalproddingthat

madetheorbitsofSaturnandJupiterlesscircular.Inturn,theorbitsofNeptune

andUranuswereperturbedandmovedoutwards,alsobecomingmoreelliptical.

NeptunecouldhaveevenbeguninsidetheorbitofUranusandthenovertakenit

inamigrationoutward,andbothplanets,particularlythefartherone,would

havescatteredsmallicybodies,sometowardstheinnerSolarSystem.

Meanwhile,thegravityandmovementofJupiterwouldflingsomeasteroidsinto

theinnerSolarSystemandpushothersaway.Duringthistime,theEarthmust

havesufferedevenmoreimpactsthantheMoonbecauseofEarth’sgreatersize

andgravity.Eventually,afterwreakinghavoc,theorbitsofthegiantplanets

wouldhavesettleddown.

Theoriginoflife

Quitehowlifearoseisunknown.ItmayhaveoriginatedonEarthoritwas

carriedherebyspacedustormeteorites.Thelatterideaiscalledpanspermia,

anddoesn’tsolvehowlifeoriginated,butpushestheproblemelsewhere.Also,

theremaybedifficultieswithsurvivaloverlongtransittimesiflifecamefrom

aroundotherstars.Forthesereasons,we’llconcentrateonaterrestrialoriginof

life.

Thereiswideagreementthattheoriginoflifewouldhavebeenprecededbya

periodofchemicalevolution,orprebioticchemistry,duringwhichmore

complexorganicmoleculeswereproducedfromsimplerones.Theideagoes

backtothe19thcentury.In1871,CharlesDarwinimaginedthatsuchchemistry

mighthaveoccurredina‘warmlittlepond’,accordingtoalettertothebotanist

JosephHooker:

It is often said that all the conditions for the first production of a living organism are now

present,whichcouldeverhavebeenpresent.—Butif(andohwhatabigif)wecouldconceive

in some warm little pond with all sorts of ammonia and phosphoric salts,—light, heat,

electricityetc.present,thataproteincompoundwaschemicallyformed,readytoundergostill

more complex changes, at the present day such matter would be instantly devoured, or

absorbed,whichwouldnothavebeenthecasebeforelivingcreatureswereformed.

Thus,Darwinconcludedthatlifeisunlikelytooriginatetodaybecause

organismsarecontinuallyeatingthechemicalcompoundsthatareneeded.On

theotherhand,beforelifeexisted,chemicalconditionsfortheoriginoflife

wouldhavebeenprevalent.

Inthe1920s,theRussianbiochemistAlexanderOparinandtheBritishbiologist

J.B.S.Haldanebothrecognizedthattheenvironmentunderwhichlifearose

wouldhavelackedoxygen.Earth’soxygen-richatmosphereisaproductof

photosynthesisbyplants,algae,andbacteria.Anoxygen-freeatmospherewould

havebeenbettersuitedtoprebioticchemistrybecauseoxygenconvertsorganic

matterintocarbondioxide,whichpreventsthebuild-upofcomplexmolecules.

Indeed,acontemporarygeologist,AlexanderMacGregor,reportedin1927that

hehadfoundsedimentaryrocksdatingfromwithintheArchaeanAeon(3.8–2.5

Ga)thatshowedthattheancientatmospherelackedoxygen.Inparticular,

MacGregorobservedthatironmineralsinwhatisnowZimbabwewere

unoxidized,unliketherust-colouredironoxidesinmodernsedimentsthatare

producedwhenatmosphericoxygenreactswithiron-containingminerals.Today,

MacGregor’sdeductionissupportedbymanyotherdata(Chapter4).

OparinandHaldanealsoproposedthatgasesinEarth’searlyatmospherewould

havebeenconvertedbyultravioletsunlightorlightningintoorganicmolecules.

Inthe1950s,suchideasweretestedwhenStanleyMiller,astudentoftheNobel

Prize-winningchemistHaroldUrey,devisedanexperimentattheUniversityof

Chicago.Ablendofammonia,methane,hydrogen,andwatervapourwasputin

aflaskandelectricsparkspassedthroughthegastosimulatelightning.Miller

foundthatyellowishwatercondensedoutatthebottomoftheflask.Thisdark

residuecontainedorganicmolecules,includingvariousaminoacids—the

buildingblocksofproteins.Atthetime,theMiller–Ureyexperimentwas

trumpetedinthemediaasvirtuallysolvingtheproblemoftheoriginoflife.Life

wassaidtostartina‘primordialsoup’generatedbyatmosphericchemistry.In

1953,whenMillerwroteuphisresults,acommonview(datingbacktoOparin

andHaldane)wasthatgeneticmaterialwasprotein,butamonthbeforeMiller’s

papercameout,DNAwasidentifiedastherealbasisofheredity.Subsequently,

experimentsintheMiller–Ureygenrehavealsoproducedmoleculescontaining

hexagonalringsofcarbon,nitrogen,andhydrogenatoms,whicharethekindof

ringsfoundinDNA.

However,geochemistshaveraiseddoubtsabouttheMiller–Ureyexperiment

becausetheyarguethattheEarth’searlyatmospherewasprobablynotas

hydrogenrichasMillerassumed.VolcanoessupplytheEarth’satmospherewith

gasesoverlongtimescalesbutmostvolcanicgasissteam,i.e.watervapour

O),withanaverageoflessthan1percenthydrogen(H

).Similarly,volcanic

carboncomesoutascarbondioxide(CO

)ratherthanthehydrogenatedformof

methane(CH

),whilenitrogenisemittedasdinitrogen(N

)insteadofammonia

(NH

ItisthoughtthattheatmosphereintheHadeanwasmostlymaintainedfrom

gasesreleasedthroughvolcanism,sotheairshouldhaveconsisted

predominantlyofCO

andN

.Ofcourse,itdependsonhowfarbackyougo.

WhentheEarthwasforming,thevaporizationoflargeimpactorswouldhave

producedhydrogen-richatmospheresifimpactorsweresufficientlyrichiniron

toprovidetherightkindofchemistrytostabilizehydrogenintheimpact

explosion.Earlyon,mostoftheatmospherewassteamandoceanshadnotyet

condensed.Afteroceansformed,atmospheresproducedbyimpactvaporization

wouldhavebeenephemeral.Becauseofuncertaintiesaboutthecompositionof

theHadeanatmosphere,astrobiologistshaveproposedsourcesoforganiccarbon

otherthanthatpresumedbyMillerandUrey.

Onealternativeisthatorganiccarboncamefromspace.Somecarbon-rich

meteoritescontainaminoacids,alcohols,andotherorganiccompoundsthat

perhapscouldhaveseededEarthwiththematerialsneededforprebiotic

chemistry.Forexample,theMurchisonmeteorite,whichfellonMurchison,

Australia,in1969,containsover14,000differentmolecules.Also,

micrometeoriteparticlesthatare0.02–0.4mminsizecurrentlybringabout30

millionkgoforganiccarbontotheEarthperyear.Organiccarboncanfallintact

totheEarth’ssurfacebecausetinyparticlesdon’tnecessarilyburnupinthe

atmosphere.Furthermore,theinteriorsofcometsarerichinorganicmaterialand

mayhaveplayedasimilarroletometeoritesingettinglifestartediftheorganic

materialsurvivedcometaryimpactsontheEarth.

Anotherpossiblesourceoforganiccarbonisdeep-seahydrothermalvents.A

hydrothermalventisaspotwherehotwateremergesfromtheseafloor.New

seaflooriscreatedatmid-oceanridges,wheretheEarth’stectonicplatesseparate

andmagmarisesupfromthemantle,fillingthevoid.Seawaterflowsdown

throughcracksneartheridges,isheatedbythehotmagma,andrises,pickingup

substancesproducedbychemicalreactionsbetweenthewaterandrocks,suchas

hydrogen,hydrogensulphide,anddissolvediron.Whenthehotacidicwaterhits

cold(2°C)oceanwater,itproducesaplumeofprecipitatedparticlescontaining

dark-colouredpyrite,anironsulphidemineral(FeS

,whereFeisironandSis

sulphur).Thatiswhythesehydrothermalventsarecalled‘blacksmokers’.One

hypothesissuggeststhatlifeoriginatedonthesurfaceofsuchpyriteminerals.

However,blacksmokersareveryhot,around350°C,somostresearcherswho

favouradeep-seaoriginoflifehavefocusedonventsthatarecooler,around

90°C,alkalineratherthanacidic,andlocatedfurtherfromthemid-oceanridges.

Beneathalkalinevents,hydrogenreleasedinreactionsbetweenwaterandrock

cancombinewithcarbondioxidetomakemethaneandbiggerorganic

molecules.IntheHadean,tinyporesinthemineralstructuresthatgrewupfrom

ventscould,inprinciple,havecontainedandconcentratedsimplemoleculesto

allowthemtoreactandmakemorecomplexprebioticmolecules.

Also,inalkalinevents,becausethefluidemergingismorealkalinethan

seawater,thereisanaturalgradientinpHthatissimilartothatincells.Iflife

originatedinsuchanenvironment,itmightexplainwhyenergyproductionis

intimatelytiedtopHgradients.

Lifegeneratesenergyfrommicroscopicelectricalmotorsthatareembeddedin

cellmembranesandrunoffelectricalcurrentsdrivenbypHgradientsacrossthe

membranes.Itisimpossibleforwordstodojusticetotheseamazingmolecular

machines,soIsuggestthatthereadersearchtheInternetfor‘ATPsynthase

animations’.Essentially,metabolicenergyisusedtocreateadifferentpHon

eachsideofacellmembranewiththeoutsideusuallymoreacidicthanthe

inside.Thisdifferentialisaprotongradient,becausepHisaninversemeasure

oftheconcentrationofpositivelychargedhydrogenions(protons)insolution—

thegreatertheconcentration,thelower(andmoreacidic)thepH.Theproton

gradientisessentiallyabattery.Whendischarged,electricalcurrentflows

throughthemolecularturbineinthecellmembraneandgeneratesmoleculesthat

storeenergy.Thesemoleculesofadenosinetriphosphateor‘ATP’aretheenergy

carriersinallcells.Forexample,ahumantypicallycontains250gofATPand

usesupabodyweight’sworthofATPeverydayasATPismadeused,and

remade.

TheaboveprocessofgeneratingATPiscalledchemiosmosis,whichcontainsthe

word‘osmosis’,meaningwatermovingacrossamembrane,becausetheproton

movementisanalogous.Chemiosmosisissoweirdcomparedwiththefamiliar

ideaofgeneratingenergyfromchemicalreactionsthatwhenitwasproposedin

1961byBritishbiochemistPeterMitchellmostbiochemistswerehostile.

Eventually,MitchellwasvindicatedwithaNobelPrizein1978.Giventhat

chemiosmosisisuniversaltoterrestriallifeandpresumablyrelatedtoitsorigin,

itisinterestingtowonderwhetheriongradientsmightbefundamentalforlife

anywhere.

Currently,thesourceoftheorganiccarbonthatledtotheoriginoflife—the

atmosphere,space,orhydrothermalvents—isunresolved.Givensuchasource,

however,organiccompoundsmusthavebeenconcentratedforchemical

reactionstohappen,whichcouldhaveoccurredwhenfilmsformedonmineral

surfaces,perhapsarisingfromdryingorcoldness.Thenthesimpleorganics

couldjoinuptomakemorecomplexmolecules.Butchemicalsystemsneed

severalotherfeaturesbeforetheycanbeconsidered‘alive’.Themainonesarea

metabolism,enclosurebyasemi-permeablemembrane,andreproductionthat

incorporateshereditywithagenome.

Withregardtothelastcharacteristic,amoretractableissueforlaboratory

experimentationthantheoriginoflifeitselfisahypothesizedstageofveryearly

life,calledtheRNAWorld,inwhichRNAprecededDNAasthegeneticmaterial.

Inmoderncells,DNA(deoxyribonucleicacid)providesaninstructionsetthatis

transcribedintoRNA(ribonucleicacid)moleculesthatcarrytheinformation

neededtomakespecificproteins.DNAissimilartoRNAbutmorecomplicated,

soitislogicalthatRNAevolvedfirst.Moreover,inthe1980sitwasdiscovered

thatsomeRNAmolecules,knownasribozymes,canactascatalysts.Soitis

plausiblethatRNAoncecatalyzeditsself-replicationandassemblyfromsmaller

molecules.Atthenextstage,RNAwouldbegintomakeproteins,someofwhich

wouldbebettercatalyststhanRNAitself.Eventually,DNAwouldreplaceRNA

becauseitismorestableandcanbelarger,bothofwhichconferreproductive

advantages.

Anothercriticalfeatureintheoriginoflifewastheencapsulationofagenomein

acellmembrane.Thiswasbeneficialforacoupleofreasons:first,toincrease

ratesofreactionbyconcentratingbiochemicals;andsecond,togivean

evolutionaryadvantagetoself-replicatingmolecules.Forexample,anRNA

genomethatproducedausefulproteincouldkeepittoitselfinsideacell,

therebyfavouringitsprogeny.

So-called‘pre-cell’structureshavebeenmadeinthelab.Sphericalstructures

formspontaneouslywhenoilsaremixedwithwater,whichissomethingwe’re

allfamiliarwithinthekitchen.Suchoilyorfattymembranesplausiblyledtothe

membranesthatsurroundmoderncellsandweresurfacesforprebiotic

chemistry.

Subsequenteventsmayhaveoccurredasfollows.RNAtookupresidenceinpre-

cells,cellswithanRNAgenomeevolved,andthenmoderncellswithaDNA

genometookoverfromRNA.Atsomepoint,metabolismalsoevolved,perhaps

afterRNAWorld.However,thereremainsanongoingdebateaboutwhichcame

first,metabolismorgeneticreplication.Also,theactuallocation,conditions,and

evolutionarystepsfortheoriginofliferemainuncertainandareanareaof

researchripeforsignificantprogress.

Chirality(ortheartofclappingwithonehand)

Adescriptionoflife’soriginwouldbeincompletewithoutmentioningarelated

aspectofbiochemistry:forbiomoleculesthatcomewithtwomirror-image

structures,lifeonEarthusesonlyoneofthestructures,notboth.Thepropertyof

havingmirror-imagesymmetryiscalledchirality,whichcomesfromtheGreek

cheirfor‘hand’,giventhefactthataperson’shandsaremirrorimagesand

cannotfitontopofoneanother(thesamewayup).Ifyou’renotconvincedthat

aleft-handstructureistrulydifferentfromarightone,trywearingyourshoeson

thewrongfeetallday!

Chiralityarisesinbiomoleculesifacentralcarbonatomissurroundedbyfour

differentgroupsofatoms.Thegroupsnaturallyarrangethemselvesina

tetrahedron,whichisapyramidwithfourtriangularfaces.Ifwenumberthe

groupswitha‘1’atthetopofthetetrahedron,thethreetetrahedralfeetcanbe

numberedclockwiseorcounterclockwiseasshown:

Thetwomoleculesinthepictureare‘handed’becauseonecannotbe

superimposedontopoftheother.Suchmoleculesarecalledenantiomersfrom

theGreekenantiosmorphe,foroppositeshape.Let’sconsideraspecific

enantiomer,theaminoacidalanine.Itwouldhave‘1’asahydrogenatom,‘2’as

acarboxylgroup(COOH),‘3’asanaminegroup(NH

),and‘4’asamethyl

group(CH

).Byconvention,withtheclockwisenumberingofgroups2to4,

alanineissaidtobeintheright-handedorD-formaftertheGreekdextrofor

right.Withthecounterclockwisearrangement,alanineisintheLformfromthe

Greeklaevoforleft.Remarkably,lifeonEarthusesL-aminoacidsinproteins,

almostentirely,andD-sugarsalmostexclusivelyaswellwhetherinDNA,cell

walls,orsynthesis.(Ahandymnemonicis‘lads’forleftamino,dextrosugars).

However,whenachiralsubstanceismadeinthelaboratory,equalproportionsof

left-andright-handedformsareusuallyproduced,calledaracemicmixture.

Chiralmoleculescanhaveverydifferenteffectsdespitetheiridenticalchemical

formulae.Anexampleistheinfamousdrugthalidomide.Theright-handedform

curesmorningsicknessinpregnantwomen,buttheleft-handedversioninduces

severebirthdefects.Amorepleasantexampleistheflavouringlimonene:theD-

formtasteslikelemonsbuttheL-formisorange-flavoured.

Whatcausedthecommonchirality,orhomochirality,sharedbyalllife?There

aretwogeneralhypotheses.Oneisthattheorganicmoleculesfromwhichlife

startedhadaslightexcessofoneenantiomerthatwassubsequentlyamplified.

Physicalprocessesinvokedtoinducethisexcessincludepolarizedlightfrom

starsthatmightcauseorganicmoleculesthatseededtheEarthtodevelopachiral

biasduringtheirsynthesisinspace;alternatively,polarizedradiationfrom

radioactivedecaymightgenerateanenantiomericexcess.Aproblemwithsuch

radiation-relatedideasisthattheexcessisoftensmall,lessthan1percent.The

secondhypothesisisthatprebioticchemistrypreferentiallyassembledonlyone

oftheenantiomers.Forexample,adsorptiononamineralsurfacemighthave

chiralselectivity.Chemicalkineticsisgenerallymoreefficientforapure

substancethanamixture,sothismightcausefurtheramplification.One

biologicalprocess—reproductionofagenome—mayrequirehomochirality.

HomochiralitywasprobablyneededforthereplicationintheRNAWorldmodel

becauseonlyreplicatedRNAstrandsofthesamechiralitywouldlineupwithan

RNAtemplatestrand.

Signsoftheearliestlife

Althoughtheoriginofliferemainsobscure,intheoldestsedimentaryrockson

Earthwefindpossiblesignsoflife.Perhapsit’snocoincidence,buttheserocks

(mixedwithvolcanicflows)datefromtheendoftheLateHeavyBombardment,

around3.8Ga.TheyarefoundinIsua,intheinteriorofsouth-westGreenland.

Theyformadark,forebodinglandscapethatlookssomethinglikeMordor,albeit

icy,andtheyareremarkablefortellingusmuchabouttheearlyEarth.Thereis

gravelthatwasroundedbytheactionofwater.Also,somesedimentsthatwere

laiddownundertheseacontainflakesofcarbonintheformofgraphite.Isotopic

analysissuggeststhatthiscarbonwasoncepartofmicrobeslivingintheoceans.

Carbonhastwostableisotopes,carbon-12andcarbon-13.Lifetendsto

concentrateafewpercentmorecarbon-12atomsinitstissuesthancarbon-13

becausemoleculescontaininglightercarbon-12atomsreactfasterin

biochemistry.ThegraphiteinIsuaisenrichedincarbon-12byabout2percent,

whichissimilartotheproportioninmarinemicrobes.Sotheinferenceisthat

marinemicrobesdiedandsankintothesedimentswherethecarbonwas

subsequentlycompressedandconvertedintographite.

Betterpreservedevidenceforearlylifeisfoundinnorth-westAustraliaand

datesfromabout3.5Ga.Inthe1980s,mycolleagueRogerBuickfoundfossil

stromatolitesofthisagenearNorthPoleinnorth-westernAustralia(Fig.2).An

Aussiejokernamedtheplace‘NorthPole’becauseitisoneofthehottest,most

sun-bakedlocationsonEarth.Stromatolitesarelaminatedsedimentarystructures

madebyphotosynthesizingmicrobialcommunitiesinwaterthatisshallow

enoughtoreceivesunlight.Oftenstromatolitestaketheformofdomesof

wrinklylaminations.Thesedomesarebuiltupfromtrappedsedimentassheets

ofmicrobes,calledmicrobialmats,growingupwardstowardssunlight.On

modernstromatolites,thelivingpart—themat—isjustaveneerabouta

centimetrethickwithatextureliketofuthatsitsatopaccumulatedmineral

layers.IfyoucouldtravelbacktotheArchaeanEarth,youwouldseecoastlines

coveredinstromatolitesallovertheworld.Today,stromatolitesarerarebecause

certainfishandsnails(whichdidn’texistintheArchaean)feedonthemicrobial

matsthatmakethem.Consequently,modernstromatolitesarefoundonlyin

lagoonsorlakeswherethewateristoosaltyforsuchvoraciousanimals.

2.Left:Cross-sectionoftheworld’soldestfossilstromatolitesintheDresser

Formation,NorthPole,Australia.Right:Aplanviewofthebeddingplane

ofthestromatolites,showingtheirtops.Thelenscapis5cmdiameterfor

scale

SomescepticswonderiftheNorthPolestructurescouldhaveformedwithout

lifeandsoarenotreallystromatolites,butmostastrobiologistsaccepttheir

organicheritagebecausetheyhavevariousfeaturesweexpectofbiology.The

fossilstromatoliteshavelaminationswithextremelyirregularwrinklesthatare

difficulttoexplainasfoldscreatedbypurelyphysicalprocesses.Also,the

laminationsthickenatthetopofconvexflexures,whichisjustwhathappens

whenphotosyntheticmicrobesgrowwherethereismoresunlight.Introughs

betweenflexurestherearefragmentsthatareplausiblyinterpretedaspiecesof

microbialmatthatwererippedupbywavesinthesea.Thefragmentscontain

thin,wrinklylayersthatincorporateorganiccarbon.Thecombinationofsuch

observationssuggeststhattheNorthPolestromatolitesareindeedtheoldest

fossilstructuresvisibletothenakedeye.

Tofindevidenceforpastlife,wecanalsolookforindividualdeadbodiesof

single-celledorganisms,sinceonlymicrobesexistedsofarbackinEarthhistory.

Suchmicrofossils,whichcanonlybeseenwithamicroscope,arefoundin

sedimentaryrockssuchaschert(afine-grainedsilica-richrock;flintisafamiliar

example)andshales,whicharefine-grainedsedimentaryrocksthatwereonce

muds.

Geologistswhostudymicrofossilstakerocksamplesfromlikelysettingsbackto

thelabwheretheyslicethemupandhopethatsomethingshowsupunderthe

microscope.It’sahit-and-missaffair.Theproblemofdistinguishinglifefrom

non-lifealsorearsitsuglyheadagainbecausemineralgrainssometimeslook

likefossilcells.Consequently,microfossilsaremoreconvincingiftheyshow

signsofcelldivision,colonialbehaviour,orfilamentousstructuresthatoccur

whencellsjoinupinaline.Microfossilscanalsobetestedtoseeiftheycontain

organiccarbon.

TheoldestincontestablemicrofossilsoccurinSouthAfricafrom2.55Ga.They

areconvincingbecausetheyarefoundinfossilizedmatsoffilamentsthat

containorganiccarbonandincludesomemicrobialformsimmobilizedintheact

ofcelldivision.ThereareolderpossiblemicrofossilsinSouthAfricadatedat

3.2–3.5Gathatarespheroidal,containorganiccarbon,andappeartoshowcell

division,buthavenootherbiologicalattributes.Similarmicrofossilcoloniesin

3.4GasandstoneoftheStrelleyPoolFormationinnorth-westAustraliaalso

possessorganiccarbonwithisotopescharacteristicoflife.

Theworld’smostcontroversialmicrofossilscomefromtheApexChertrock

formationinnorth-westAustralia,withanageof3.5Ga.Thesewereidentified

asthe‘world’soldestmicrofossils’inthe1990sbyBillSchopf,oftheUniversity

ofCalifornia,LosAngeles.Thestructuresappearasblacktodarkbrownstreaks

thatappeartobepartitionedintocell-likesections.SchopfgavethemLatin

namesimplyingthattheywereextinctcyanobacteria,whicharephotosynthetic

bacteria.In2002,arancorousdisputearosewhenMartinBrasierofOxford

Universityre-examinedthesamplesandshowedthattheydidn’tlooklike

microbialfilamentsinthreedimensions.Brasieralsonotedthatthechertwasnot

sedimentarybutatypeproducedinahydrothermalvent,whichisanunlikely

placetofindcyanobacteriathatneedsunlight.Sincethen,furtherinvestigation

hassuggestedthatthefilamentsarereallyfracturesthatarepartiallyfilledwith

haematite,anironoxide,whichprovidesthedarkcolour.However,organic

carbonisdispersedinthechertoutsidethefilamentsandthecontroversy

continues.

Sofar,we’vediscussedisotopes,microfossils,andstromatolites.Further

possibleindicatorsofearlylifearebiomarkers,whicharerecognizable

derivativesofbiologicalmolecules.We’reallfamiliarwiththeconceptthat

fossilskeletonsallowustodistinguishaTyrannosaurusrexorTriceratops.On

themicroscopicscale,microbescanleavebehindremnantsofindividualorganic

molecules,consistingofskeletalringsorchainsofcarbonatoms.These

‘molecularbackbones’comefromparticularmoleculesthatarefoundonlyin

certaintypesofmicrobe.Sobiomarkersnotonlyconfirmthepresenceofpast

lifebutcanalsoindicatespecificformsoflife.

Unfortunately,likesomemicrofossils,ancientbiomarkersarealsomiredin

controversy.Theoldestreportedbiomarkersdatefromabout2.8Gaandappear

toshowmoleculessuggestingthepresenceofoxygen-producingcyanobacteria.

Butthesampledrocksmayhavebeencontaminatedbyyoungerorganic

material.Morecertainbiomarkersarefoundinrocksfrom2.5Gainsidetiny

fluidinclusionsthatappearuncontaminated.

Despitethelimitationsofsomeevidence,theoverallrecordshowsthatEarth

wasinhabitedby3.5Ga—within1billionyearsofitsformationandshortlyafter

heavyimpactbombardment,whichsuggeststhatlifemightoriginatefairly

quicklyongeologicaltimescalesonsuitableplanets.Thismeansthatit’snotout

ofthequestionthatMars,whichwethinkhadageologicallyshortwindowof

habitability(Chapter6),mayalsohaveevolvedlife.

Chapter4

Fromslimetothesublime

HowdidEarthmaintainanenvironmentfitforlife?

Howeverlifestarted,onceestablished,itpersistedforover3.5billionyearsand

evolvedfrommicrobialslimetothesophisticationofhumancivilization.During

thisperiod,theEarthmaintainedoceansand,forthemostpart,amoderate

climate,eventhoughtherewasanincreaseofabout25percentintheamountof

sunlight.ThegradualbrighteningoftheSunisaconsequenceofthewaythatthe

Sunshinesonthemainsequence.Whenfourhydrogennucleiarefusedintoone

heliumnucleusintheSun’score,therearefewerparticles,somaterialabovethe

corepressesinwardstofillavailablespace.Thecompressedcorewarmsup,

causingfusionreactionstoproceedfaster,sothattheSunbrightensabout7–9

percenteverybillionyears.ThistheoryisconfirmedbyobservationsofSun-

likestarsofdifferentages.

IftheEarthhadpossessedtoday’satmosphereatabout2Gaorearlier,thewhole

planetshouldhavebeenfrozenunderthefainterSun,butgeologicalevidence

suggestsotherwise.ThispuzzleisthefaintyoungSunparadox.Thereis

evidenceforliquidwaterbacktoatleast3.8Ga,whichincludes,forexample,

thepresenceofsedimentaryrocksthatwereformedwhenwaterwashedmaterial

fromthecontinentsintotheoceans.

TherearethreewaystoresolvethefaintyoungSunparadox.Themostlikely

explanationinvolvesagreatergreenhouseeffectinthepast.Anothersuggestion

isthattheancientEarthasawholewasdarkerthantodayandabsorbedmore

sunlight,althoughthere’sscantsupportingevidence.Athirdideaisthatthe

youngSunshedlotsofmaterialinarapidoutflow(solarwind)tospacesothat

theSun’scorewasn’tascompressedandheatedovertimebyoverlyingweight

asassumedabove.Ifthemasslosswasjustright,theSuncouldhavestartedout

asbrightastoday.However,observationsofyoungSun-likestarspresentlydon’t

supportthethirdhypothesis.

Inconsideringthefirstidea,weneedtoappreciatethattheatmospherewarms

theEarthtoanextentdependingonatmosphericcomposition.Withoutan

atmosphere(andassumingthattheamountofsunlightthattheEarthreflects

stayedthesame),theEarth’ssurfacewouldbeachilly–18°C.Instead,today’s

averageglobalsurfacetemperatureis+15°C.The33°Cdifference(=18+15)is

thesizeofEarth’sgreenhouseeffect,whichisthewarmingcausedbythe

atmosphere.

Howdoesthegreenhouseeffectwork?Aplanet’ssurfaceisheatedbyvisible

sunlight,causingittoglowintheinfraredjustasyourwarmbodyshinesatthose

wavelengths.Anatmospheretendstobemostlyopaquetotheinfraredradiation

comingupfromthesurfacebelow,soitabsorbstheinfraredenergyandwarms.

Becausetheatmosphereiswarmitalsoradiatesintheinfrared.Someofthis

radiationfromtheatmospheretravelsbackdowntotheplanet.Sothesurfaceof

aplanetiswarmerthanitwouldbeintheabsenceofanatmospherebecauseit

receivesenergyfromaheatedatmosphereinadditiontovisiblesunlight.

At3.5Ga,whentheSunwas25percentfainter,a50°Cgreenhouseeffectwould

havebeenneededtomaintainthesameglobalaveragetemperatureonEarththat

weenjoytodaywitha33°Cgreenhouseeffect.Astrongergreenhouseeffectis

possibleiftherehadbeengreaterlevelsofgreenhousegases,whicharethose

responsibleforabsorbinginfraredradiationcomingfromtheEarth’ssurface.

Today,watervapouraccountsforabouttwo-thirdsofthe33°Cgreenhouse

effect,andcarbondioxide(CO

)accountsformostoftherest.However,water

vapourcondensesasrainorsnow,soitsconcentrationisbasicallyaresponseto

thebackgroundtemperaturesetbyatmosphericCO

,whichdoesn’tcondense.In

thisway,CO

controlstoday’sgreenhouseeffecteventhoughitslevelissmall.

Around1700,therewereabout280partspermillionofCO

inEarth’s

atmosphere(meaningthatinamillionmoleculesofair,280wereCO

),whilein

2010therewere390partspermillion.Sinceindustrialization,CO

hasbeen

releasedfromdeforestationandburningfossilfuelssuchasoilorcoal.Inthe

lateArchaean,anupperlimitontheCO

amountoftenstoahundredtimes

higherthanthepre-industriallevelisdeducedfromthechemicalanalysesof

palaeosols,whicharefossilizedsoils.ButevensuchlevelsofCO

werenot

enoughtocounterthefaintyoungSun.

Infact,theArchaeanatmospherehadlessthanonepartpermillionofmolecular

oxygen(O

),whichimpliesthatmethane(CH

)wasanimportantgreenhouse

gas.Today,atmosphericmethaneisatalowlevelof1.8partspermillion

becauseitreactswithoxygen,whichisthesecondmostabundantgasintheair

at21percent.(Mostoftoday’sairisnitrogen(N

),whichis78percent,but

neitheroxygennornitrogenaregreenhousegases.)IntheArchaean,thelackof

oxygenwouldhaveallowedatmosphericmethanetoreachalevelofthousands

ofpartspermillion.Methanewouldn’taccumulatewithoutlimitbecauseitcan

bedecomposedbyultravioletlightintheupperatmosphere.Subsequent

chemistryinvolvingmethane’sdecompositionfragmentscangenerateother

hydrocarbons,i.e.chemicalsmadeofhydrogenandcarbon,includingethanegas

)andasmogofsootyparticles.Acombinationofmethane,ethane,and

carbondioxide—plusthewatervapourthatbuildsupinresponsetothe

temperaturesetbythesenon-condensablegreenhousegases—wouldhave

providedenoughgreenhouseeffecttooffsetthefainterSun.Thisassumesthat

therewasasourceofmethanefrommicrobiallife,justliketoday,whichis

plausiblebecausethemetabolismformethanegenerationisancient(seeChapter

5).

Ofcourse,wecouldaskwhattheEarth’searlyclimatewaslikebeforelife.In

thatcase,CO

probablycontrolledthegreenhouseeffect,astoday.Infact,

throughoutmuchofEarthhistory,therehasbeenageologicalcycleofCO

that

regulatedclimateontimescalesofaboutamillionyears.(Itstilloperatestoday

butisfartooslowtocounteracthuman-inducedglobalwarming.)Essentially,

atmosphericCO

dissolvesinrainwaterandreactswithsilicaterocksonthe

continents.Thedissolvedcarbonfromthischemicalweatheringthentravels

downriverstotheoceans,whereitendsupinrocksontheseafloor,suchasin

limestone,whichiscalciumcarbonate(CaCO

).Ifdepositionofcarbonateswere

allthatwerehappening,theEarthwouldloseatmosphericCO

andfreeze,but

thereisamechanismthatreturnsCO

totheatmosphere.Seafloorcarbonatesare

transportedonslowlymovingoceanicplatesthatdescendbeneathotherplatesin

theprocessofsubduction.AnexampletodayistheSouthPacific‘Nazca’plate,

whichisslidingeastwardsunderChile.Carbonatesaresqueezedandheated

duringsubduction,causingthemtodecomposeintoCO

.Volcanism(where

rocksmelt)andmetamorphism(whererocksareheatedandpressurizedbut

don’tmelt)releaseCO

.ThewholecycleofCO

lossandreplenishmentis

calledthecarbonate–silicatecycleandbehaveslikeathermostat.Iftheclimate

getswarm,morerainfallandfasterweatheringconsumeCO

andcooltheEarth.

IftheEarthbecomescold,CO

removalfromthedryairisslow,soCO

accumulatesfromgeologicalemissions,increasingthegreenhouseeffect.

Thecarbonate–silicatecycleprobablyregulatedclimatebeforelifeoriginated.It

thenlikelyplayedanincreasinglyimportantroleasathermostatafterlevelsof

methanegreenhousegasdeclinedintwostepswhenatmosphericoxygen

concentrationsincreased,firstaround2.4Gaandthen750–580Ma(Ma=

millionsofyearsago).

AcaveatisthatthecyclemayhaveoperateddifferentlyintheHadeanand

possiblytheArchaeanbecauseplatetectonics—thelarge-scalemotionof

geologicplatesthatrideonconvectioncellswithinthemantlebelow—probably

hadadifferentstyle.Radioactiveelementsinthemantlegenerateheatwhenthey

decay,andmoreweredecayingontheearlyEarth.So,ontheonehand,ahotter,

lessrigidHadeanmantleshouldhaveallowedoceaniccrusttosinkmore

quickly.Ontheotherhand,ahottermantleshouldhaveproducedmoremelting

andthicker,warmeroceaniccrustthatwaslesspronetosubduct.Overall,the

presenceofgranitesinferredfromzircons(Chapter3)impliesthatcrustmust

havebeenburiedsomehowbecausegraniteisproducedwhensunkencrust

melts.However,exactlyhowtectonicsoperatedontheearlyEarthremainsan

openquestion.

TheGreatOxidationEvent:asteptowardscomplex

life

ThemostdrasticchangesoftheEarth’satmosphericcompositionhavebeen

increasesinoxygen,whichwerejustasimportantfortheevolutionoflifeas

variationsingreenhousegases.FormostofEarth’shistory,oxygenlevelswere

solowthatoxygen-breathinganimalswereimpossible.TheGreatOxidation

Eventiswhentheatmospherefirstbecameoxygenated,2.4–2.3Ga.However,

oxygenlevelsonlyreachedsomewherebetween0.2to2percentbyvolume,not

today’s21percent.Largeanimalswereprecludeduntilaround580Maafter

oxygenhadincreasedasecondtimetolevelsexceeding3percent(Fig.3).

Nearlyallatmosphericoxygen(O

)isbiological.Atinyamountisproduced

withoutlifewhenultravioletsunlightbreaksupwatervapourmolecules(H

intheupperatmosphere,causingthemtoreleasehydrogen.Netoxygenisleft

behindwhenthehydrogenescapesintospace,therebypreventingwaterfrom

beingreconstituted.Butabioticoxygenproductionissmallbecausetheupper

atmosphereisdry.Instead,themajorsourceofoxygenisoxygenic

photosynthesis,inwhichgreenplants,algae,andcyanobacteriausesunlightto

splitwaterintohydrogenandoxygen.Theseorganismscombinethehydrogen

withcarbondioxidetomakeorganicmatter,andtheyreleaseO

aswaste.

3.Theapproximatehistoryofatmosphericoxygen,basedongeologic

evidence(ppm=partspermillion;ppt=partspertrillion)

Another,moreprimitive,typeofmicrobialphotosynthesisthatdoesn’tsplit

waterorreleaseoxygenisanoxygenicphotosynthesis.Inthiscase,biomassis

madeusingsunlightandhydrogen,hydrogensulphide,ordissolvedironin

hydrothermalareasaroundvolcanoes.Today,microbialscumgrowsthiswayin

hotsprings.

Beforeplantsandalgaeevolved,theearliestoxygen-producingorganismswere

similartomoderncyanobacteria.Cyanobacteriaarebluish-greenbacteriathat

teemintoday’soceansandlakes.DNAstudiesshowthatacyanobacteria-like

microbewastheancestorofplants,algae,andmoderncyanobacteria.

Consequently,wemightsupposethattheatmospherebecameoxygenatedonce

cyanobacteriaevolved.However,evidencesuggeststhatcyanobacteriawere

producingoxygenlongbeforeitfloodedtheatmosphere.Aplausibleexplanation

isthatreductants,whicharechemicalsthatconsumeoxygen,atfirstrapidly

overwhelmedtheoxygen.Reductantsincludegasessuchashydrogen,carbon

monoxide,andmethanethatcomefromvolcanoes,geothermalareas,and

seafloorvents.Distinctiveiron-richsedimentaryrockscalledbandediron

formationsdatingfromtheArchaeanshowthattherewasconsiderabledissolved

ironintheArchaeanocean,unliketoday’socean,whichwouldhavealsoreacted

withoxygen.

Thefirstsignsofphotosytheticoxygenappearabout2.7–2.8Gaaccordingto

evidencefromstromatolitesandthepresenceofchemicalsthatbecamesoluble

byreactingwithoxygen.Innorth-westAustralia,stromatolitesinrockscalled

theTumbianaFormationonceringedancientlakes.Intheory,microbesmight

havebuiltsuchstromatolitesusinganoxygenicphotosynthesis,butthere’sno

evidenceforhydrothermalemissionsneededforthismetabolism.Instead,

cyanobacteriausingoxygenicphotosynthesisprobablyconstructedthe

stromatolites,consistentwithtuftsandpinnaclesinthestromatolitesthatare

producedwhenfilamentsofcyanobacteriaglidetowardssunlight.Furthermore,

molybdenumandsulphur,whichareelementsonlysolublewhenoxidized,

becomeconcentratedinseafloorsedimentsafter2.8Gatolevelsthatare

possibleifmicrobesoxidizedtheseelementsonlandusinglocalsourcesof

oxygensuchasstromatolites.

Anotherreasonwhyoxygendidn’timmediatelyaccumulateintheEarth’s

atmosphereisthatitsproductionismostlyazero-sumprocess.Whena

moleculeofoxygenismade,anaccompanyingmoleculeoforganiccarbonis

generated,assummarizedinthefollowingequation:

carborndioxide+water+sunlight=organicmatter+oxygen

+H

O+sunlight=CH

O+O

Theprocesseasilyreverses,i.e.organicmatterreactswithoxygentoregenerate

carbondioxideandwater.Also,theArchaeanatmosphere’slackofoxygen

meantthatmicrobescouldreadilyconvertorganiccarbonintomethanegas,as

happenstodayinsmelly,oxygen-freelakeorseafloorsediments.Intheair,

methanecouldreactwiththeoxygentorecreatewatervapourandcarbon

dioxide.Inbothcases—directreversalorindirectcancellationwithmethane—

nonetoxygenisproduced,despitethepresenceofphotosynthesis.

However,atinyfraction(about0.2percent,today)oforganiccarbonisburied

insedimentsandseparatedfromoxygen,whichpreventsthetwofrom

recombining.EveryorganiccarbonthatgetsburiedisequivalenttoonenetO

moleculethat’sfreedup.Ofcourse,this‘freed’oxygencaneasilyreactwith

manyothersubstancesbesidesorganiccarbon,includinggeologicalgasesand

dissolvedmineralssuchasiron.Nonetheless,atippingpointwasreachedwhen

theflowofreductantstotheatmospheredroppedbelowthenetflowofoxygen

associatedwithorganiccarbonburial,causingtheGreatOxidationEvent.

Afterwards,aplateauof0.2–2percentoxygenwasreachedprobablybecause

oxygenthatdissolvedinrainwaterreactedsignificantlywithcontinental

minerals,preventingitsfurtheraccrual.

Indeed,asurgeinoxidationistheevidencefortheGreatOxidation.‘Red

beds’—rust-colouredcontinentalsurfacesthatarisewhenironmineralsreact

withoxygen—appear.Today,reddishlandsurfacesarecommon,suchasinthe

Americansouth-west.Achangeinsulphurprocessingintheatmospherealso

occurred.BeforetheGreatOxidation,whentherewaslessthanonepartper

millionofatmosphericoxygen,red-andyellow-colouredparticlesofelemental

sulphurliterallyfelloutofthesky.Theyformedinchemicalreactionsatmany

kilometresaltitudewhensulphurdioxidegasfromvolcanoeswasbrokenupby

ultravioletlight.Thefallingsulphurparticlescarriedasulphurisotope

compositionintorocksthatindicatesatmosphericformation.AftertheGreat

Oxidation,oxygencombinedwithatmosphericsulphur,preventingelemental

particlesfromforming.Theskyclearedandtheisotopicsignaturevanishesfrom

sedimentaryrocks.

WiththeGreatOxidation,Earth’sstratosphericozonelayerformed.Thisregion

ofconcentratedozonebetween20and30kilometresinheightshieldstheEarth’s

surfacefromharmfulultravioletrays.Iftherewerenoozonelayer,youwouldbe

severelysunburnedintensofseconds.Ozoneisamoleculeofthreeoxygen

atoms(O

),whichcomesfromoxygen.Anoxygenmolecule(O

)issplitby

sunlightintooxygenatoms(O),whichthencombinewithotherO

moleculesto

makeozone(O

ThecauseoftheGreatOxidationisdebatedbutmustboildowntoeithera

globalincreaseinoxygenproductionfrommoreorganicburialoradecreasein

oxygenconsumption.Theproblemwiththefirstideaisthatorganicmatter

extractsthelightcarbonisotope,carbon-12,fromseawater,butseawatercarbon,

recordedinancientlimestones,doesn’tsteadilydecreaseincarbon-12content.

Thatleavesthesecondidea.

Butwhatwouldhavecausedtheflowofreductantstodiminish?Onepossibility

isthattherelativelylargeabundancesofhydrogen-bearinggasessuchas

methaneandhydrogen(whichareinevitableinanatmospherewithoutoxygen)

meantthathydrogenescapedrapidlyfromtheupperatmosphereintospace.The

lossofhydrogenoxidizedthesolidplanet.Oxidizedrockshavefewerreductants

sothisgraduallythrottledfurtherreleaseofreductants.Oxidationhappens

becauseyoucangenerallytraceescapinghydrogenbacktoitssourceinwater

O),evenifhydrogenatomsgothroughmethane(CH

)orhydrogen(H

)

intermediaries.Sowhenhydrogenleaves,oxygengetsleftbehindwherethe

hydrogenoriginated.Watervapouritselfhastroublereachingtheupper

atmospherebecauseitcondensesintoclouds,butotherhydrogen-bearinggases

suchasmethanedon’tcondenseandsneakthehydrogenoutintospace.

AroughanalogyfortheGreatOxidationisanacid-basetitrationofthesort

performedinhighschoolbydrippingacidintoanalkalinesolution.Suddenly,

thesolutionchangescolour,typicallyfromcleartored.Theancientatmosphere

reachedasimilartransitionfromhydrogenrichtooxygenrich.Ratherthanacid

base,wecallsuchatitrationa‘redoxtitration’becauseit’sacompetition

betweenreductantssuchashydrogenandoxidantssuchasoxygen.

Aboringbillionyearsendedbytheadventofanimal

life

TheoceanandlandadjustedtothechangeoftheGreatOxidation,andbyabout

1.8Ga,atmosphericoxygenhadsettleddownandremainedbetween0.2and2

percentforanamazinglylongtime.Theevolutiontowardscomplexlifewas

slowperhapsbecauseanoxicwatersthatwereoftenrichindissolvedironor

hydrogensulphideunderlayamoderatelyoxygenatedsurfaceocean.Such

anoxicconditionsaretoxicforcomplexlife.Infact,evolutionwassosluggish

thattheintervalfrom1.8to0.8Gaiscalledtheboringbillion.Accordingtoone

scientificpaper,‘neverinthecourseofEarth’shistorydidsolittlehappentoso

muchforsolong’!

Actually,severalnotableeventsdidoccurintheotherwiseboringbillionandjust

beforeit.Around1.9Ga,thefirstfossilsofsingleorganismsappearthatare

visibletothenakedeye.Theseimpressionsofspiralcoilsofafewcentimetres

diameter(namedGrypania)wereprobablyseaweeds.InChina,acritarchs,a

typeoffossillargerthan0.05mminsizewithorganic-walls,occurat1.8Ga.

Somearethoughttohavebeenalgalcysts,whichformwhenalgaeturninto

inactiveballsduringdryperiods.Newlineagesalsoemerged,includingred

algaeat1.2Ga,whichwerelikelycapableofsexualreproduction.Today,we

boilredalgaedescendantstomakeagar,whichisusedtothickenicecream,and

weuseonetype,nori,towrapsushi.Becausesexwasdiscovered,theboring

billionwasnotthatboring.

Startingaround750Ma,andculminatingafterfitsandstartsabout580Ma,

atmosphericoxygenroseasecondtimetolevelsexceeding3percent,andthe

deepseabecamefullyoxygenated.Animalbiomarkersandtinyfossils(possibly

sponges)datefromaround630Ma.Butit’sonlyafter580Mathatwefind

complexfossilsfromcentimetrestometresinsize.Thesearestrangesoft-bodied

organismswithoutmouthsormusclesthatmusthavereliedupondiffusionof

nutrientsthroughtheirskin.Someresemblepizzaswhileotherslooklikeplant-

likefronds.However,theorganismslivedtoodeeplyintheseatoreceive

sunlightsotheycannothavebeenplants.Collectively,theyarecalledthe

EdiacaranbiotabecausetheywerefirstclearlyidentifiedintheEdiacaranHills

insouthernAustralia.TheEdiacaranslastedtensofmillionsofyearsbefore

dyingout.

Then,theCambrianExplosionoccurred,whichwastherelativelyrapid

appearanceofanimalfossilswithnewbodyarchitecturesina10-to30-million-

yearintervalafterthebeginningoftheCambrianPeriodat541Ma.

Representativesofmostmoderngroupsofanimalsemerged,includingmany

withhardskeletons.TheCambrian(541–488Ma)comesdirectlyaftertheendof

ProterozoicAeon,andisthefirstperiodofthePhanerozoicAeon(541Mato

present).Phanerozoicmeans‘visibleanimals’fromtheGreekphaneros(visible)

andzoion(animal).OnetinyCambriancreature,Pikaia,wasasortofswimming

worm,afewcentimetreslong.Pikaiawasprobablyinagroupfromwhichall

thevertebrates,includingus,evolved.

Thesecondriseofoxygenallowedadiversityofanimalstoarisebutitscause

remainsunsolved.Suggestionsincludeorganiccarbonburial(andhenceoxygen

production)acceleratedbyseveralfactors.Thesewerenewlifeonlandthat

boostedthebreakdownofsurfacesandnutrientrelease,theevolutionofmarine

zooplankton,andtheenhancedproductionofclaysthathelpedtoburyorganic

matter.Alternatively,moderatelevelsofaround100partspermillionof

atmosphericmethanethroughoutthe‘boringbillion’mighthavepromotedyet

morehydrogenescapetospacethatoxidizedtheseafloorandusheredinan

increaseofoxygenafterasecondglobalredoxtitration.

Around400Ma,athirdriseofoxygentookplacewhenvascularplants

colonizedthecontinents.Suchplantshavespecialtissues(suchaswood)to

transportwaterandminerals.Theyprobablyenhancednutrientrelease(by

breakingdownlandsurfaceswithroots)andorganicburial.Afterwards,

atmosphericoxygenlevelsremainedwithin10percentto30percent,allowing

thepersistenceofanimalsandeventuallyourownappearance.

SnowballEarthorWaterbeltEarth?

Curiously,worldwideglaciationsareassociatedwithboththeGreatOxidation

Eventandthesecondriseofoxygen,likebookendsencompassingthe‘boring

billion’.Somescientistsarguethatatthesetimes,theEarth’soceansentirely

icedoverinaso-calledSnowballEarth.Thisstatewasfarmoreextremethan

theiceagesthathaveoccurredinthelastfewmillionyearsthathaveonly

extendeddowntomid-latitudes,notthetropics.Forcontext,theProterozoic

aeonisdividedintothreeeras,thePalaeoproterozoic(2.5–1.6Ga),

Mesoproterozoic(1.6–1.0Ga),andNeoproterozoic(1.0–0.541Ga).Geologists

commonlyspeakofthe‘Palaeoproterozoicglaciations’todescribethreeorfour

glaciationsthatoccurredatthebeginning(2.45–2.22Ga)ofthe

Palaeoproterozoic,whilethe‘Neoproterozoicglaciations’referstotwolarge

glaciations(at715and635Ma)andonesmallone(at582Ma).

Therockrecordprovidesevidenceoftheseworldwideglaciations.Theflowof

icesheetsmixesstones,sand,andmudintoatypeofrockcalledtillite.Glaciers

alsoleaveparallelscratchmarkscalledstriationsonunderlyingrocks.

Furthermore,rocksdropoutoficebergsormeltingiceshelvesandendupbelow

asso-calleddropstonesthatdeformsedimentarybeds.Together,tillites,glacial

striations,anddropstonesaredeadgiveawaysthaticesheetswereoncepresent.

Magneticmeasurementsmadeinthe1980sand1990salsodemonstratedthatthe

glaciationsextendedintothetropics.AcompassshowstheEarth’snorth–south

magneticfield.Lessobviousisthefactthatthemagneticfieldlinesare

approximatelyperpendiculartothesurfacenearthepolesandparalleltothe

surfaceneartheequator.Volcanicrockscontainironminerals,suchasmagnetite

(anironoxide,Fe

),whichbecomemagnetizedinthedirectionofthe

prevailingmagneticfieldwhentheycool.Thus,whenthetrappedorremnant

magneticfieldlinesareparalleltoancientbedsofrock,weknowthattherocks

formedinthetropics.Thiswasdiscoveredinrocksassociatedwith

NeoproterozoicandPaleoproterozoicglaciations.

TheideathattheentireEarthmightfreezeoverifseaicereachedthetropics

originatedinthe1960s.ARussianclimatologist,MikhailBudyko,calculated

thatifpolaricecapsextendedfartherequatorwardthan30°latitude,theentire

Earthshouldfreeze.Icereflectsalotofsunlightandsohasahighalbedo,which

isthefraction(between0and1)reflected.Today,theEarth’salbedoisabout0.3,

meaningthat30percentofthesunlightisreflectedtospace.Butseaicereflects

50percentwhenbareor70percentifcoveredinsnow.Adisastrousice-albedo

runawayoccurswhenhigh-albedoiceinthetropicsmakestheEarthabsorbless

sunlightandcool,whichcreatesmoreicethatmakestheEarthevencolder,and

soon.Afrozenglobeiscreatedwithanaveragetemperaturebelow–35°Cand

anicethicknessof1.5kmattheequatorand3kmatthepoles.

JoeKirschvinkoftheCaliforniaInstituteofTechnologyproposedhowa

SnowballEarthwouldmelt,aswellastheveryphraseSnowballEarth.He

suggestedthatvolcanoeswouldpunchthroughtheiceandventcarbondioxide

intotheatmosphere.Becausetherewouldbenorainfallorsignificantopen

oceantodissolvecarbondioxide,itwouldbuildupoverafewmillionyears

untilanenhancedgreenhouseeffectmeltedtheSnowball.Also,theflowofice

fromthickpolaricetothintropicalicewouldcarryvolcanicashandwindblown

dust,sothatthetropicswouldbecomedarker,makingtheSnowballproneto

melt.Thentheice-albedorunawaywouldruninreverse:lessicewouldmakethe

Earthwarmer,whichwouldmeltmoreice,andsoon.

Layersofcarbonaterockcalledcapcarbonatesthatsitontopoftheglacial

depositspossiblyattesttothisaftermathofaSnowballEarth.Theideaisthat

aftertheicemelted,largeamountsofcarbondioxidewereconvertedintocap

carbonates.ProponentsoftheSnowballEarthhypothesisnotethattheratioof

carbon-12tocarbon-13isotopesinthecarbonateissimilartothatinvolcanic

carbondioxide.

AcentralproblemfortheSnowballEarthhypothesisishowphotosyntheticalgae

survivedunderthethickice,giventhattheyrequiresunlight.Lineagesofgreen

andredalgaewentstraightthroughtheNeoproterozoicsnowballsunscathed.

Onesuggestionisthatheataroundvolcanoesallowedsomeopenwaterorthin,

transparentice.ButothersquestionwhethertheentireEarthreallyfrozeover.

SomereasonablysophisticatedcomputersimulationsoftheEarth’sancient

climateproduceatropicalbeltofopenwater—aWaterbeltEarth—becausethis

regionissurroundedbybareiceofmoderatealbedo,whereastheveryreflective,

snow-coverediceoccursonlyathighlatitudes.

Whateverhappened,thecauseofboththeNeoproterozoicandPaleoproterozoic

glaciationswasprobablyadiminishedgreenhouseeffect.Continentsdrift

becauseofplatetectonics,andreconstructionshavethembunchedupinthe

tropicspriortotheancientsnowballeras.Rainfallovertropicalcontinents

perhapsdrewcarbondioxidedowntolowerlevelsthantypical,poisingtheEarth

forglaciation.Also,thecoincidenceofrisesinoxygenwiththeglaciationeras

suggestsatriggerthroughmethane,anothergreenhousegas.Probablythe

oxygenincreaseswereassociatedwithrapiddecreasesinmethanebecausehigh

abundancesofthetwogasesareincompatible—theyreact.

Forastrobiology,thelessonfromSnowballEarthisthatthebiosphereonan

Earth-likeplanetisresilient.Despitedrasticclimateswings,lifeonEarth

survivedandemergedaftertheNeoproterozoicintoanewaeonofanimals.

Ontheoccurrenceofadvancedlife

Inastrobiology,weoftenwonderifanimal-likelifeexistselsewhereinthe

galaxy.ThatpossibilitycanperhapsbeinformedbyevolutiononEarth.To

becomecomplex,terrestriallifehadtoovercomeatleasttwokeyhurdles.First,

lifehadtoacquirecomplexcells,whichwereneededfordifferentiationonthe

largerscale,i.e.cellswithfunctionaldistinctions,suchasliverversusbraincells.

Large,three-dimensionalmulticellularanimalsandplantsaremadeonlyof

eukaryoticcells,whichcandevelopintoamuchlargerrangeofcelltypes

comparedtothecellsofmicrobes(seeChapter5).

Asecondprecursorforanimalswashavingsufficientoxygentogeneratelotsof

energywithanaerobic(oxygen-using)metabolism,suchasourown.Abundant

energyisneededtogrowlargeandmove.Anaerobicmetabolism,whichdoesn’t

useoxygen,producesabouttentimeslessenergyforthesamefoodintakethan

aerobicmetabolism.Forextraterrestriallife,wemightwonderiffluorineor

chlorine—whicharepowerfuloxidants—couldbeusedinplaceofoxygento

generatehighenergylevels.Theanswerisno.Fluorineissoreactivethatit

explodeswithorganicmatter,whilechlorinedissolvestomakeharmfulbleach.

Oxygen,itappears,isthebestelixirforcomplexlifeintheperiodictable—

reactiveenoughbutnottooaggressive.

Oxygennotonlyneedstobepresentbutalsoconcentrated.Thefirstaerobic

multicellularlifeprobablyconsistedofaggregatesofcellsproducedwhen

dividingcellsfailedtoseparate.Suchagglomerationswouldhavebeenlimited

insizebydiffusionofoxygentotheinnermostcellsothatgreaterlevelsof

externaloxygenwouldhavepermittedlargercellgroupings.Attypical

metabolicrates,adiffusion-constrainedanimalprecursorofaboutafew

millimetres’dimensionwouldneedatmosphericoxygenconcentrations

exceeding3percent.Thiscorrespondstotheoxygenincreasearound580Ma

thatusheredintheanimals.

Fossilalgaeshowthateukaryotesexistedlongbeforeanimals,sotheslowfuse

totheCambrianExplosionwasprobablythetimeittooktobuildupadequate

oxygen.Wecanevendefineanoxygenationtimeasthatrequiredtoreach

oxygenlevelssufficientforcomplexanimallife.OnEarth,itwas4billionyears

aftertheplanetformed.

TheoxygenationtimeonEarth-likeexoplanetsisuncertaingiventhatwe’restill

tryingtounderstandwhatsetthetimescalefortheEarth.However,oxygen

comesfromliquidwater.SoEarth-likeexoplanetswithoceanshavethepotential

todevelopoxygen-richatmospheresifwater-splittingphotosynthesisevolves.If

theoxygenationtimeexceeds12billionyearsonacertainexoplanetbecauseit’s

endowedwithmaterialthatreactswithoxygen,aSun-likeparentstarwouldturn

intoa‘redgiant’beforetheconditionsforanimallifebecamepossible,andthe

planetwouldbedoomedtopossessnothingmorecomplexthanmicrobialslime.

Butifotherexoplanetshaveshortoxygenationtimes,complexlifemightoccur

fasterthanittookonEarth.

Trendsinevolution?Thelessonofmassextinctions

Onceanimalsevolve,wemightwonderifevolutionhasatrendtowardsgreater

complexity.Considerlifeonland.Inlandcolonization,thefirstplantsporesare

foundaround470Ma,whilefossilsofsubstantialpartsofplantsappearabout

425Ma.Insectsappearonlandaround400Ma.Thenfishevolvedinto

amphibiansaround365Maandtheirdescendantsultimatelybecamethereptiles

andmammals,includingus.Thisisatrendforthebiospheretousemoreofthe

Earth’sresources.Butamisconceptionistothinkofevolutionasasteady

progressionendinginus.Thefossilrecordshowsthatspeciescomeandgo.

Also,massextinctionsintermittentlyprunethediversityofcomplexlife.They

areeventswhenmorethan25percentoffamiliesarelost,where‘family’isthe

biologicaltierabovegenusandspecies(seeChapter5).Suchmassextinctions

refertothenon-microbialpartofthebiosphere,ofcourse.

OnlyaroundtheProterozoic–Phanerozoictransitionisthereadramaticincrease

indiversitybecauseatthattimeorganismsacquirednewbodyarchitecturesand

waysoflivingthatpersisted.Inanimals,akeyinnovationwasthebodycavity,

thecoelom(pronounced‘see-lum’),whichcouldbefilledwithfluidtoprovide

rigidityandallowtheconcentrationofforcesfrommuscles.Thisfacilitatedself-

propulsion,whichwasfurtherimprovedwithhardskeletons,atfirstexternaland

theninternal.Apredator–preyevolutionaryarmsracelikelycontributedtothe

CambrianExplosionofanimalvarieties.Plantdiversitylaterincreasedwiththe

developmentofvascularsystemswithrigidcellsfortransportandassociated

organsthateventuallygaverisetotrees.

Inthepast500Ma,therehavebeenfivemassextinctionsthatkilledover50per

centofspecies,withthetwobiggestat251Maand65Ma,respectively.

Excludingcrocodilians,nolandanimallargerthanthesizeofadomesticdoggot

throughthelargestevent,separatingthePermian(299–251Ma)andTriassic

(251–200Ma)periods.Thiseventtookamassivetollintheoceans.For

example,thetrilobites—iconsoftheCambrianseas—hadbeenindeclinebutthe

Permian–Triassicextinctionfinishedthemoff.Thesecondlargesteventat65

MawipedoutthedinosaursandseparatestheCretaceous(145–65Ma)and

Paleogene(65–23Ma)periods.(Someliteraturereferstothiseventas

Cretaceous-Tertiarybasedonanoldernamingsystem.)ThePermian–Triassic

massextinctiondestroyedasmuchas95percentofmarinespeciesand70–80

percentoflandanimals,whiletheCretaceous–Paleogeneextinguished65–75

percentofallspecies.

ThecauseofthePermian–Triassicmassextinctionwasapparentlyachainof

misfortunesgeneratedbytheEarthherself.Thetriggerseemstohavebeenlarge-

scalevolcanisminSiberia,coveringanareasimilartoEurope.Becausecoal

depositsunderlaythisarea,thevolcanismpumpedouthugeamountsofcoal-

derivedmethaneandcarbondioxidegreenhousegases,whichwarmedand

acidifiedtheocean.Oxygenislesssolubleinwarmwater,soananoxicdeepsea

developed,whichmayhavebelchedpoisonoushydrogensulphidetothesurface.

Thecombinationofclimaticwarming,oceanacidification,andnoxiousgases

extinguishedmorelifethananyothereventinthePhanerozoic.

Theimpactofanasteroidofabout10kilometresdiameterappearstohave

causedtheCreataceous–Paleogenemassextinction.Calamitousconsequences

includedtemporarydestructionoftheozonelayer,worldwidewildfires,acid

rain,andsubsequentclimaticcoolingcausedbysulphateparticlesinjectedinto

thestratospherethatreflectedsunlight.Theimpactcraterhasevenbeenfound

aboutakilometreundergroundneartheMexicantownofChicxulub

(pronounced‘sheek-soo-loob’).

Extinctionsdestroypreviouslysuccessfullineagesbuttheyalsoprovide

opportunitiesforothers.YouarereadingthisbookbecauseoftheChicxulub

impactor.Themammalsbecamedominantoncethedinosaursweregone.

Ontheotherhand,massextinctioneventsmeanthatifacivilizationdevelops,it

canbewipedout.ImpactsthesizeofthatproducingtheCretaceous–Paleogene

extinctionshouldoccurevery100millionyearsorsoonEarth,giveortakea

factorofafew.Theimplicationisthatcivilizationsonexoplanetscouldbeshort-

livedcomparedwiththeageoftheuniversemerelybecauseofsuchrandom

catastrophes.Thus,anEarth-likeplanetcanremainfitforlife,butcivilizations

areprobablyephemeral.Thelatterhasconsequencesintheastronomicalsearch

forextraterrestrialintelligence(Chapter7).

Chapter5

Life:agenome’swayofmakingmoreand

fittergenomes

LifeonEarth:theviewfromabove

InChapter1,Ipresentedageneraldefinitionoflife,butinfindinglife

elsewhere,ithelpstoknowouroneexampleoflifeingreatdetail.Tothisend,

let’sstartwithaglobalperspectiveofterrestrialbiologyandworkdownwardto

cellsandmolecules.

ImagineaninterstellartravellerwhoarrivesontheEarthandwantstoknow

aboutourbiology.Perhapsfromherplanet,shehaddeducedthatlifeexistshere

becauseofEarth’swet,anomalouslyoxygen-richatmosphere,orshehadpicked

upTVbroadcastsfromdecadesagothatsomehowdidn’tputheroffvisiting.

Amazingly,shespeaksEnglish(asextraterrestrialsalwaysdointhemovies)and

byastrokeofluckherspacecraftlandsinanEnglish-speakingcountry!What

wouldwetellher?

Atthegloballevel,Earth’sbiosphereisthesumofalllivinganddead

organisms.Sometimesthetermincludesthenon-livingregionsthatlife

occupies.Byquantifyingthebiosphereinbillionsoftonnesofcarbon(1tonne=

1,000kg),wecanidentifyitsbroadcomponents.Thebiomassonlandisaround

2,000billiontonnesofcarbonofwhich30–50percentislivingandtherestis

dead.Intheocean,only0.1–0.2percentofabout1,000billiontonnesof

biomasscarbonisalive.Forestsarethereasonthatthere’ssomuchmoreliving

biomassonlandthanintheoceans.

Anunsettledissuethat’srelevanttolifeelsewhere(belowthesurfaceofMarsor

Jupiter’smoon,Europa)istheextentofEarth’ssubsurfacebiosphereor

‘intraterrestriallife.’Somescientistssuggestthatahugemassofmicrobes

extendsakilometreortwobelowtheseafloorandmorethan3kilometres

underneathland.Alimitforlifeatsuchdepthsistemperature.Asyougo

downwards,itgetswarmer(asminersknow)and,atsomepoint,toohotforeven

thetoughestmicrobes.Earth’ssubsurfacebiospherebiomassisuncertain

becausedeepdrillinghasnotbeendoneforalltypesofsubsurface

environments,butestimatesrangefromabout1percentto30percentofEarth’s

livingbiomass.

Whateveritsprecisemass,thebiosphereislessthanabillionthofthemassof

theEarthandyetmanagestogreatlyinfluencethechemistryofthesurface

environment.Thebiospherecandothisbecauseitprocessesvastamountsof

materialwithrapidturnover.Indoingso,individualsliveanddiealmost

instantlyongeologicaltimescales.Microbestypicallyreproduceintensof

minutestodays,whilelargemulticellularorganismslastonlyafewthousand

yearsatmostbeforetheybecomedeadfodderformicrobialdegradation.For

example,amongstthelatter,theoldestnon-clonalorganismisaBristleconePine

intheWhiteMountainsofCalifornia,whichgerminatedaround3049BC,several

centuriesbeforetheearliestEgyptianpyramids,butamereblipingeological

time.

Themainactivitiesintoday’sbiosphereareoxygenicphotosynthesisandits

chemicalreversalthroughaerobicrespirationandoxidation(Chapter4).

However,microbespossessahugerangeofothermetabolisms,whichwe

discusslater.

Microbes,ofcourse,arefoundalmosteverywhereattheEarth’ssurface.A

seawaterandfreshwaterbacteriumcalledPelagibacterubiqueisprobablythe

mostnumerousorganismonEarth.Despitethat,itwasfirstdescribedonlyin

2002,whichshowshowbiologyisstilldeveloping.Overall,thereareabout10

microbesofallvarietiesintheocean,farexceedingthe10

starsinthe

observableuniverse.Microbesarealsoabundantonland.Therearetypically

about100millionto10billionmicrobespergrammeoftopsoil.Indoorair

usuallyhasaboutamillionbacteriapercubicmetre.Anaverageofonemicrobe

(albeitdead)evenfloatsinevery55cubicmetresofairat32kilometresaltitude

inthestratosphere.

TherearefourkeypropertiesthathaveallowedtheEarthtobecomeso

extensivelyinhabited.Themostimportantiswidespreadliquidwater.All

metabolizingorganismscontainorganicmoleculesdispersedinaqueous

solution.Consequently,wedon’texpectlifetoexistonacompletelydrysurface,

suchasVenus.Thesecondessentialpropertyishavingenergyformetabolism.

SunlightisthemainsourceforEarth’sbiosphere,butsomeorganismsobtain

energyfromchemicalreactionsindarkness,whichmeansthatit’snotimpossible

thatmicrobial-likelifemightexistbelowthesurfaceofMarsorEuropa.Athird

life-givingfeatureisarenewablesupplyofessentialchemicalelements.Aplanet

thatcan’tresupplyvitalelementsthroughnaturalcycles(suchasthewatercycle

ortectonics)wouldbedeathly.Afourthattribute,whichmaybeessentialfor

life,isthepresenceofinterfacesbetweensolids,liquids,andgases.It’s

advantageoustoliveatastableinterface,suchasonlandorthesurfaceofthe

ocean.ThisiswhyitwouldbedifficultforlifetoexistonagasgiantlikeJupiter

thathasnosurface.Lifemightspeculativelybepossibleatcertainaltitudesin

Jupiter’satmospherebutdeepchurningbyconvectionwouldperiodicallyplunge

lifeintoaninterioroffatalheatandpressure.

Aninsideviewofterrestriallife:thecell

Wehavejustconsideredbiologyonaglobalscalebutunderthemicroscopeall

organismsarecomposedofcellsofdifferenttypesthatputthemintooneofthree

domains,theEukarya,Archaea,orBacteria.Thelasttwoaremicrobialandare

sometimeslumpedtogetherasprokaryotes,althoughmanymicrobiologistsnow

considerthistermantiquatedbecausearchaeaandbacteriaarebiochemically

dissimilar.DNAfloatsfreelyinthemiddleofthearchaealandbacterialcells,

whereasineukaryotestheDNAishousedinsideamembrane-boundnucleus.

Archaeaandbacteriaaresinglecells,withtheexceptionofsomebacterial

speciesthatjoinupinarowtoformfilaments.Eukaryotescanbesinglecelled,

suchasanamoebaorabaker’syeast,butonlyeukaryotesformlarge,three-

dimensionalmulticellularorganisms,suchasmushroomsorhumans.

Theclassificationintothreedomainsoflifewasmotivatedbygeneticsand

supersedesanolder‘fivekingdom’systemofplants,animals,fungi,protists

(single-celledeukaryotes),andbacteria.However,theseoldtermsarestillused

intaxonomy,whichclassifiesanorganismbelowitsdomainaccordingto

Kingdom,Phylum,Class,Order,Family,Genus,andSpecies.ThemnemonicI

usetoremembertheselevels(ortaxa)isfartoorudetomentionbutanotheris

‘KeepingPreciousCreaturesOrganizedForGrumpyScientists’.Thetaxonomic

levelsofahuman,forexample,aretheanimalkingdom,chordatephylum,

mammalclass,primateorder,hominidfamily,Homogenus,andsapiensspecies.

TheSwedishbotanistCarolusLinnaeus(1707–78),whogaveustheword

biology,developedmoderntaxonomy,includingbinomialnamesfororganisms,

e.g.Homosapiens.Butittooktheadventofevolutionarytheoryandmolecular

biologytouncoverthebiochemicalunityoflifeandgeneticcommonancestry.

Whiletherearesomesimilaritiesbetweeneukaryotesandtheothertwodomains,

there’salsoagulfincomplexity.Bacteriaandarchaeaareusuallyaround0.2–5

microns(millionthsofametre)insize,withrareexceptions,whereaseukaryotic

cellsaregenerallybigger,at10–100micronssize.Thelargereukaryoticcells

containorganellestoperformspecializedfunctions,analogoustotheorgansof

thehumanbody(Fig.4).Forexample,themitochondriacarryoutrespiration.In

plantoralgalcells,chloroplastsperformphotosynthesis.However,onefeature

commontoallcellsisalargenumberofribosomes,whichareglobular

structuresthatmakeproteins.Forexample,inprokaryotesorsimpleeukaryotes

suchasyeast,thecellmighthaveseveralthousandribosomes,whileinan

animalcellthenumbermightreachseveralmillion.

4.a)Schematicofprokaryote(archaeaandbacteria)versuseukaryote

structure;b)Twobacteriacaughtintheactofconjugation

Givenanastrobiologicalinterestincomplexextraterrestriallife,wemightask

whyonlytheeukaryoticcellproduceslarge,three-dimensionalmulticellularlife.

Theanswerisnotfullyknownbuteukaryoticcellshaveamoresophisticated

internaldynamiccellskeletonorcytoskeletonthanarchaeaorbacteria.This

consistsofproteinmicrofilaments,tinyproteintubes(‘microtubules’),and

molecularmotorsthatcontrolcellstructureandhelptransportsignalling

moleculestochangecellphysiology.Sotheabilitytodevelopintomany

specializedforms(forexample,skinorbraincells)isinherentinthemake-upof

aeukaryoticcell.Cyanobacteriaareabletomakefilamentsofhundredsofcells

inarowwithsomedifferentcelltypesbutthatisthelimit.Unlikearchaeaor

bacteria,eukaryoteshavebiggerandmoremodulargenomes,whichalsoallows

formorecomplexity.Iftherewerenoeukaryoticcells,theEarthwouldbemuch

duller.Allofthefamiliarorganismsofourworld—theanimals,plants,andfungi

—wouldn’texist.Thus,whenwethinkaboutcomplexlifeonexoplanets,we

shouldwonderwhetherevolutionwouldmakecellslikeeukaryoteselsewhere.

Forthisreason,theoriginofeukaryotesisofgreatinterest.

Moderngeneticsimpliesthattheeukaryoticcellisa‘Frankenstein’smonster’,

assembledinevolutionaryhistoryfrombitsandpiecesofbacteriaandarchaea.

Forexample,themitochondrionineukaryoticcellswasderivedfroma

bacteriumoriginallylivingsymbioticallyinsideanothercell.Thelargercell,

whichmayhavebeenanarchaeonorsome‘proto-eukaryote’thatnolonger

exists,swallowedthefree-livingbacteriumand,inthemostimportantgulpin

history,themitochondrialancestorcameintobeing.Indeed,separateDNAin

mitochondriaprovidesevidenceofbacterialancestry.ThedistinctDNAof

chloroplastsinplantandalgalcellsshowsthatchloroplastswerederivedina

similarwayfromsymbioticcyanobacteriathatendeduplivinginsidelarger

cells.Effectively,allthecellsintheleavesofgreenplantscontainancestorsof

cyanobacterialslavesthatwerecagedandco-optedlongago.Thetheoryofsuch

anoriginforthemitochondria,chloroplasts,andotherorganellesineukaryotic

cellsiscalledendosymbiosis.

Aworldwithouteukaryoteswouldalsobeonewithoutsex.Thinknoflowersor

lovesongs.Archaeaandbacteriaarenotsexedbuttheydoconjugatewhentwo

cellsareconnectedbyatubeinwhichgenesaretransferredinpiecesofDNA

calledplasmids.Bacterialsurfaceshaveprotuberancescalledpili,andduring

conjugationaspecialpilusextendstoapartner,providingtheconduit(Fig.4).

Unlikesexualreproductionineukaryotes,microbialconjugationdoesn’tproduce

offspringandisquickandeasy.It’sasifyoubrushedupagainstapersonwith

redhairinacoffeeshopandacquiredthered-hairedgenewithaninstantchange

ofyourhaircolour.Archaeaandbacteriacanalsoacquirenewgenesthrough

transformation(uptakeofforeignDNAfromtheenvironment)andtransduction

(virus-mediatedgeneswapping).Indeed,therapidacquisitionofgenesallows

thequickdevelopmentofbacterialresistancetoantibiotics.

Eukaryotesthataresexedgenerategametes,i.e.spermandeggcells.Incomplex

multicellulareukaryotes,thediversityoflifemakesitsurprisinglyhardtodefine

‘male’and‘female’,leavinguswiththeodddefinitionthatmalesarethosethat

producethesmallgametes,whilefemalesproducethebigones.Thegametes

fusesothathalfofthegenescomefromafatherandhalffromamother.Usually

DNAisalooselycoiledthreadbutbeforecelldivision,DNAcurlsupinto

visiblechromosomesunderamicroscope.Forexample,humanshave46

chromosomesin23pairsineachcell,exceptforthegametes,whichhavehalf,

i.e.23chromosomes.

Therearemanyideasaboutwhysexisevolutionarilyadvantageousfor

eukaryotes.Onepossibilityconcernshowitmixesandmatchesgenesfromboth

parentsontoeachchromosomeinaprocesscalledrecombination.Ifbeneficial

mutationsoccurseparatelyintwoindividuals,themixtureofbothcan’tbe

achievedinasexualorganisms,butsexuallyreproducingorganismscanbring

themtogetherandreapthebenefits.Conversely,sexcanalsoeliminatebad,

mutatedgenesbybringingunmutatedgenestogetherinsomeindividuals,

whereasself-cloningorganismsarestuckwithbadgenes,andoffspringcandie

becauseofthem.

Outsidethethreedomains,virusesrepresentagreyareabetweenthelivingand

non-living.Virusesaretypicallyabouttentimesmoreabundantthanmicrobesin

seawaterorsoil.TheyconsistofpiecesofDNAorRNAsurroundedbyprotein

and,insomecases,afurthermembrane.Virusesaretiny,onlyabout50–450

nanometres(billionthsofametre)insize,comparabletothewavelengthof

ultravioletlight.Theyaregenerallyconsiderednon-livingbecausetheyare

inanimateoutsideacellandhavetoinfectandhijackcellsfortheirown

reproduction.However,somedothiswithoutthehostevernoticing,sonotall

virusescausedisease.Onetheoryofseveralfortheoriginofthenucleusof

eukaryotesisthatitmayhaveevolvedfromalargeDNAvirus,buttheroleof

virusesintheevolutionoflifeisstillamatterofdebate.

Thechemistryoflife

Todiscussmanyaspectsoflife,suchasgeneticsandmetabolism,requiresthe

vocabularyofbiochemistry.Thefourmainclassesofbiomoleculearenucleic

acids,carbohydrates,proteins,andlipids.Likeself-assemblyfurniture,many

biomoleculesaremodular.Theyarechainsorpolymersofsmallerunitscalled

monomers.

Carbohydratesprovideenergyandstructure.TheycontainC,H,andOatomsin

a1:2:1ratio,withrepeatedunitsofC(H

O),soliterally,theirchemical

compositionis‘carbonhydrated’.Sugarswithfivecarbonatomsarefoundin

DNAandRNAmolecules,whilesix-carbonsugarsexistincellwalls,suchas

celluloseinplants.

Lipidsareorganicmoleculesthatareinsolubleinwaterbutdissolveinanon-

polarorganicsolvent—onewithoutsignificantelectricalchargeonanyofits

atoms,suchasoliveoil.Thus,ifwegroundupadeadanimalandbatheditina

non-polarsolvent,anythingthatdissolvedwouldbealipid.Lifeuseslipidsin

cellmembranes,asfatsforenergystorage,andassignallingmolecules.Major

componentsofmembranesarephospholipids,whichhaveahydrophilic(water-

loving)endthatcontainsphosphorusandahydrophobic(water-repellent)tail

thatisahydrocarbon,consistingofcarbonandhydrogenatoms.Adoublelayer

ofphospholipids,calledabilayer,formsamembrane.Thehydrophilicendsstick

outintoanaqueousmediumontheinteriorandexteriorofacell,while

hydrophobictailsfaceeachotherinthemiddleofthemembrane.

Proteinsarepolymersmadeupofaminoacidunits.Theiruseincludesenzymes

andstructuralmolecules,althoughthereisalonglistofotherfunctions.

RNAandDNAarenucleicacids,whicharepolymersofnucleotidemonomers.

Eachnucleotideismadeupofafive-carbonsugar,aphosphate,andapartcalled

abase(Fig.5).InDNA,therearefourpossiblebases.Allofthemcontainoneor

tworingsofsixatomswherefouroftheatomsarecarbonandtwoarenitrogen.

EachbasehasaletterdesignationofA,C,G,andT,whichstandforadenine,

cytosine,guanine,andthyminemolecules,respectively.MoleculesofRNAuse

thesamethreebasesexceptthattheTofDNAisreplacedbyaUforuracil.

5.Left:DNAconsistsoftwostrandsconnectedtogether.Eachstrandis

madeupofa‘backbone’ofphosphate(P)andsugar(S)components.The

strandsconnecttoeachotherwithbasepairs.Right:Inthreedimensions,

eachstrandisahelix,sothatoverallwehavea‘doublehelix’

In1953,JamesWatsonandFrancisCrickfamouslydeducedthestructureof

DNA:twopolynucleotidestrandscoiledinascrew-likehelix(Fig.5).Thebases

atthesidesofeachstrandsticktogetherby‘hydrogenbonds’inwhichaslightly

positivelychargedhydrogenatomononebaseisattractedtoaslightly

negativelychargedatomonabasefromtheoppositestrand.Structural

compatibilityonlyallowsaCbasetopairwithGandanAtopairwithT.Each

DNAmoleculehasseveralmillionnucleotides,andeachchromosomeinthecell

containsaDNAdoublehelix.Incontrast,RNAmoleculesaremostlysingle

stranded.However,RNAcanfoldbackonitselfifcomplementarybasesexistin

twoseparatepartsofthestrand,notingthatadeninepairswithuracil(U)in

RNA.

ThestructureofDNAincorporatestwofundamentalcharacteristicsoflife

identifiedinChapter1:anabilitytoreproduce;andablueprintfordevelopment

andmaintenance.Inreplication,theDNAhelixsplitsintotwostrandswitheach

servingasatemplateforanewcomplementarystrand.Forexample,whereverA

appearsonthetemplate,aTisaddedtothenewlygeneratedstrand,orvice

versa.ThesameappliesforG–Cpairs.Intheprocess,mutationsandmistakes

allowforevolution.

Infulfillingitsotherroleastheblueprint,theDNAdoublehelixisunzippedby

anenzymetoprovideinstructionstogenerateproteins.Partoftheunzipped

DNAundergoestranscriptionintoastrandofmessengerRNA(mRNA),which

isacomplementarycopyoftheDNA,exceptthatU(insteadofT)isinserted

whereverAappearsintheDNA.ThenthemRNAisfedintoaribosomelikea

ribbon.Inthegeneticcode,groupsofthreeletters(calledcodons)alongthe

mRNAspecifyeachaminoacidinaproteinthatflowsoutoftheribosome.

Apartfromreproduction,lifehastosustainitselfthroughmetabolism,which

involvesbreakingdownmoleculestomakeenergy(catabolism)aswellas

buildingupbiomolecules(anabolism).Theclassificationofmetabolisms

dependsontheneedforenergyandcarbon.Becauseeachoftheserequirements

can,inturn,besatisfiedintwodifferentways,biologistshave2×2=4

metabolictermsfororganisms:chemoheterotroph,chemoautotroph,

photoheterotroph,andphotoautotroph(Fig.6).

6.Theclassificationschemeformetabolismsinterrestriallife

Allorganisms(probablyevenextraterrestrialones)fallintooneormoreofthese

categories.Thetrophsuffixmeans‘tofeed’andthetwowaysinwhich

organismsgetenergy,fromchemicalsourcesorsunlight,giverisetothechemo-

andphoto-prefixes.Anadditionalhetero-orauto-prefixisemployeddepending

uponthemethodusedtoacquirecarbon.Ifcarbonisobtainedbyconsuming

organiccarboncompounds(suchassugars),thehetero-prefixapplies.Ifan

organismconvertsinorganiccarbon(e.g.carbondioxide)intoorganiccarbon—

called‘fixingcarbon’—theauto-prefixisused.Ingeneral,heterotrophsmust

acquirefoodtomakeenergy,whileautotrophscanfixcarbonandmaketheir

ownenergy.

Wehumansarechemoheterotrophs.YouandIconsumeorganicchemicalsmade

byotherorganisms,suchasplants.Allanimalsandfungi,manyprotists,and

mostknownmicrobesarechemoheterotrophs.Incontrast,achemoautotrophisa

microbethatusesinorganicchemicalssuchashydrogen,hydrogensulphide,

iron,orammoniatomakeenergy,someofwhichitusestoextractcarbonfrom

carbondioxide.Forexample,chemoautotrophicmicrobesliveindarknessin

deep-seahydrothermalventsbyoxidizinghydrogensulphideandother

substances.(Chemoautotrophsarealsocalledchemolithotrophs,fromtheGreek

‘lithos’forstonebecausetheinorganicchemicalstheyusecomefromgeological

sources.)Plants,algae,andsomecyanobacteriaareallphotoautotrophsbecause

theyusesunlightforenergyandacquirecarbonfromtheair.Photosynthetic

bacteriafromtheChloroflexusgenus,whicharefoundinhotsprings,arean

exampleofmicrobesthatmetabolizeasphotoheterotrophsusingsunlightfor

energyandacquiringcarbonfromorganiccompoundsmadebyothermicrobes.

ChemoautotrophsmightinhabitthesubsurfaceofMarsorEuropa,while

phototrophsmightexistintheoceansofhabitableexoplanets.

Thetree(orweb)oflife

ThehistoryoflifeonEarthcanbededucedfromthewaythatevolutionhas

alteredgenes.Evolutionisthechangeininheritedcharacteristicsinapopulation

fromonegenerationtothenext.Becauseindividualsaregeneticallyvariable,in

anygivenenvironmentsomewillbebetteradaptedandhavegreater

reproductivesuccessthanothers,whichbiologistsdescribeashigherfitness.In

everygeneration,individualsoflowerfitnessarelost.Thisisnaturalselection.

So,overmanygenerations,lineagesaccumulategeneticadaptationsandnew

speciesevolve.

Insexualorganisms,speciesaregroupsthatcannotinterbreedundernatural

conditions,suchashumansandhorses.Manybacteriaandarchaealivein

differentecologicalnichesthatdefineseparategroups,buttheeasewithwhich

genescanmovebetweenmicrobesmakesitmoredifficulttodesignatespecies

ofmicrobe.Consequently,geneticsisused.Adifferenceofabout2–3percentin

thegenesthatcodetheRNAcontainedinribosomesisenoughtoseparate

microbialspecies.

7.The‘treeoflife’constructedfromribosomalRNA

Infact,wecanassesstherelatednessofalllifeformsbycomparinggenes.For

example,genesindicatethatyouandthefungusbetweenyourtoes—whichare

botheukaryotes—aremuchcloserrelativesthananarchaeonandabacterium,

despiteappearances.Fromthemid1960sonwards,varioustechniqueshavebeen

developedtoassessorganismsatthemolecularlevel,includingacomparisonof

thesequencesofeitheraminoacidsinproteinsornucleotidesinRNAandDNA.

Genesdefinethesequenceofaminoacidsinaprotein,sodifferencesinthe

‘proteinsequence’betweenspeciesindicatedisparityorrelatedness.For

example,‘cytochromec’isarespiratoryproteinof104aminoacidsfoundin

manyorganisms.Thesequencesareidenticalinhumansandchimps,showing

theircloserelationship.Comparedtothehumanprotein,arhesusmonkeyhas

onedifferentaminoacid,adoghasthirteendifferentaminoacids,andsoon.As

speciesbecomemoredistantlyrelated,theproteinsequencesdiverge.Withthis

sortofdata,ananalystcandrawupatree,likeafamilytree,thatrelatesallthe

species.

Inthe1970s,theAmericanmicrobiologistCarlWoese(1928–2012)usednucleic

acidsratherthanproteinstodeterminerelationshipsamongorganisms.He

examinedRNAinribosomes(rRNA,forshort),withthekeyinsightthatthis

enabledanalysesofalllife.Proteinsynthesis,thejobofribosomes,isacore

functionofanycell.Consequently,thegenesthatcodeforrRNAshouldhave

mutatedslowlyovertimebecausemostmutationsforthisprocesswouldhave

beenfatalandsofailedtoaccumulate.About60percentofthedryweightof

ribosomesismadeupofrRNA,andtheremainderisprotein.Ribosomesin

prokaryotesaresmallerthanineukaryotes,butotherwisesimilarinstructureand

function,allowingtheircomparison.

WoeseisolatedasmallrRNAandbycomparingthedifferencesinthenucleotide

sequences,hebuilta‘treeoflife’(Fig.7).Theconstructionoftheevolutionary

historyoforganismsinthiswayiscalledphylogeny.Woese’streewasashock

becauselifegroupedintothethreedomainsmentionedearlier,unlikethe‘five

kingdom’paradigminwhicharchaeawerelumpedwiththebacteria.

Furthermore,Woese’streedemotedplants,animals,andfungitomeretwigsat

theendoftheeukaryoticbranch.

Today,whileWoese’sconceptionofthreedomainsiswidelyaccepted,studiesof

othergenesshowthatatreewithverticaldescentfromonegenerationtothenext

istoosimple.Thisisbecausemicrobesswapgeneswilly-nilly(Fig.4),

sometimestounrelatedspecies,whichiscalledlateral(orhorizontal)gene

transfer.Thus,microbialbranchesofthetreeoflifearemorelikea‘weboflife’,

criss-crossedbylateralgenetransfers.

Sincethemid1990s,DNAsequencinghasbecomemoreautomated.For

sequencingasinglegenefromenvironmentalsamples,theprocessisasfollows:

isolateDNAfromcells→copy(or‘amplify’)aDNAgenemanytimesusinga

procedurecalledthe‘polymerasechainreaction’→obtainthegenesequence

→comparewithotherorganisms→produceaphylogenetictree.Increasingly,

wholegenomesaresequenced,includingthehumangenome,whichcontains

roughly21,000protein-codinggenes.

Astonishingly,humanprotein-encodinggenescoveronly1.5percentof3

billionnucleotides.Therestofthesequenceissaidtobe‘non-coding’or‘junk

DNA’.Thesetermsareactuallymisnomersbecausemanypartsofnon-coding

DNAregulatewhencertaingenesareexpressed(i.e.giverisetoproteins)or

codefornon-proteinproductssuchasrRNAs.

Thesurprisingdivisionbetweenarchaeaandbacteriarevealedbygenesis

confirmedbybiochemicaldifferences.Forexample,bacterialcellwallscontain

peptidoglycan,whichconsistsofcarbohydraterodscross-linkedbyproteins,

whereasarchaealcellwallshavevariablechemistryofproteinorcarbohydrate,

orboth.Eukaryotecellwalls,forcomparison,aremadeofcelluloseinplants,

chitininfungi,andarenon-existentinanimalcells,whichhaveonlya

membrane.Furthermore,thecellmembranelipidsofbacteriaandarchaeadiffer.

First,therearedissimilarchemicallinkagesbetweenthehydrophobicand

hydrophilicendsofphospholipids.Second,insteadofalipidbilayerwith

hydrophobicendsdanglingface-to-faceinthemiddleofthemembrane,archaea

havemoleculesthatconnectallthewaythrough.Thisprovidesstrengthandis

onereasonwhysomearchaeacanliveinveryhotwaterashyperthermophiles.

Therearealsoecologicaldistinctions.Forexample,noarchaeonisapathogen,

soyou’llnevergetadiseasefromarchaeawhereasyoucanfromvarious

bacteria.

Onefurtheruseofphylogenyisthatthegeneticdifferencebetweentaxacanbe

relatedtothetimeingeologicalhistorythattheydiverged,makingamolecular

clock.Inthe1960s,EmileZuckerkandlandtheNobelPrize-winningchemist

LinusPaulingcomparedproteinsfortaxathatwereknownfromfossilstohave

divergedfromacommonancestor.Theyfoundthatthenumberofaminoacid

differencesisproportionaltoelapsedtime.Oneinterpretationisthatmost

changesare‘neutralmutations’thathavenoeffectonfitness.Similarly,the

numberofnucleotidesubstitutionsincertainDNAsequencesisproportionalto

elapsedtime.Molecularclocksworkbestwithcloselyrelatedgroupsofspecies

thatarelikelytohavehadsimilarratesofmutation.Distantlyrelatedspecies

havedisparategenerationtimesandmetabolicrateswithvariablemutationrates

thatmustbetakenintoaccount.Touseamolecularclock,acalibrationpoint

fromthefossilrecordfixesthedateofaparticularancestorinacomputer

algorithmappliedtothemoleculardata.Aswegobackverydeepintotime,

therearefewerfossils,sothetechniquebecomeschallenging.Nonetheless,

molecularclocksindicatethatthelastcommonancestorofanimalsoccurred

about750–800Ma,whichiscuriousbecauseitpredatestheoldestanimal

fossils.

Despitethecomplicationsoflateralgenetransfers,thetreeoflifeprovides

informationaboutearlylife.Thermophiles(microbesthatthriveathigh

temperatures)thatgrowbestat80–110°CarefoundneartherootoftherRNA

treeinthearchaeaandbacteria(Fig.7).Areasonableinferenceisthatthelast

commonancestorlivedinahydrothermalenvironment.Organismsclosetothe

rootarealsochemoautotrophs,suggestingthatprimitivemicrobesprobably

gainedenergyfrominorganiccompounds.Thetreealsoshowsthatcomplex

organisms—theplants,fungi,andanimals—werelatetoevolve,consistentwith

thefossilrecord.Consequently,ifEarth’sphylogenyisaguideforastrobiology,

theimplicationisthesameasthefossilrecord:lifeelsewhereoughttobemostly

microbialformuchofthehistoryofthehostplanet.

Lifeinextremeenvironments

Apartfrombeingclusterednearthelastcommonancestor,thermophileswere

thestartingpointforresearchintoextremophiles.Extremophilesareorganisms

thatthriveunderenvironmentalconditionsthatareextremefromahuman

perspective.In1965,ThomasBrock,anAmericanmicrobiologist,discovered

pinkfilamentsofbacterialivingattemperaturesof82–88°Cinasteamyhot

springinYellowstoneNationalPark.Atthattime,nolifewasknowntoexist

above73°C,soBrock’sdiscoveryspurredaninterestinexploringthelimitsof

life.

Althoughitwasn’tanticipated,Brock’seffortsalsoultimatelyenabledthe

explosioningenetics.Brockfoundanewbacterium,Thermusaquaticus,in

anotherhotspring.Fromthismicrobe,industrialscientistsisolatedanenzyme

stableathightemperaturesthatwasabletocatalysethepolymerasechain

reaction(PCR)—theDNAduplicationtechniquethatrevolutionizedbiology.

Thus,purescience—inthiscase,whatwenowcallastrobiology—endedup

benefitingsocietyunexpectedly.Today,PCRtechnologyisamultibillion-dollar

industry.Brock,however,gaveawayhisbacteriumanddidn’tgetapenny.

Often,thegrowth(meaningreplication)ofextremophileseitherrequiresextreme

conditionsorisoptimalatthem.Fortemperature,theupperlimitforeukaryotes

is62°C,comparedto95°Cforbacteriaand122°Cforarchaea.Therecord

holder,themethane-producingarchaeon,ormethanogen,Methanopyrus

kandleri,hasoptimalgrowthat98°C.

Identificationofvariousextremophiles(Box1)indicatesthatlifeexistsina

muchwiderrangeofenvironmentsthananyonethoughtfeasiblefiftyyearsago,

whichopensuppossibilitiesforlifebeyondEarth.Forexample,thermophiles

mightsurvivedeepundergroundonMarsbecausetheyexistdeepintheEarth’s

crust.AnotherexampleconcernslifeinLakeVostok(250kmlong,50kmwide,

and1.2kmdeep),whichsitsbelow4kilometresoficeineastAntarctica.Ice

some100metresabovethelakeisthoughttohavefrozenfromlakewater.

Oddly,itcontainstracesoftheDNAofchemoautotrophicthermophiles.This

suggeststhatbeneaththecoldlake(at–2°Cbecauseofthehighpressure),there

maybehydrothermalwatercontainingthermophilesemanatingfromfractures.

ThetechniqueshonedfordetectinglifeinlakessuchasVostokcanbeappliedin

thesearchforlifeelsewhereintheSolarSystem.Samplingforlifeinthe

subsurfaceoceanofJupiter’smoon,Europa,involvessimilarchallenges.

Box1Extremophiles

Acidophiles:requireanacidicmediumatpH3orbelowforgrowth;some

tolerateapHofbelow0.

Alkaliphiles:requireanalkalinemediumabovepH9foroptimalgrowthand

someliveuptopH12.

Barophiles(orpiezophiles):liveoptimallyathighpressure;someliveatover

1,000timestheEarth’satmosphericpressure.

Endoliths:liveinsidetheporespaceofarock(endo=within,lithos=stone).

Halophiles:requireasaltymedium,atleastathirdassaltyasseawater(halo=

salt).

Hypoliths:liveunderstones(hypo=under).

Psychrophiles:growbestbelow15°C,whilesomegrowdownto–15°C,with

reportsaslowas–35°Cformetabolism.

Radioresistantmicrobes:resistionizingradiation,suchasfromradioactive

materials.

Thermophiles:thriveattemperaturesbetween60and80°C,whilea

hyperthemophilehasoptimalgrowthabove80°C;theycanbecontrastedwith

mesophiles,suchashumans,whichlivebetween15and50°C.

Xerophiles:growwithverylittleavailablewater,andmaybehalophilesor

endoliths(xeros=dry).

Chapter6

LifeintheSolarSystem

Whichworldsmightbehabitabletoday?

In2002,whileteachingastrobiology,Iofferedaprizetoanystudentwhocould

guessninecelestialbodiesuptotheorbitofPlutothatIreckonedmightpossibly

harbourextraterrestriallife(Table1).Noonewon.But,withthegrowthof

astrobiologydiscoveriesandonlineinformation,someonereceivedtheprizein

2010.TodayImightaddseveralmorebodies,butwe’llleavethoseuntiltheend

ofthisChapter.

ThetypeoflifethatI’mconsideringinTable1issimple,comparableto

microbes,andtheguidingprincipleconcernsliquidwater.OnEarth,wherever

wefindliquidwater,wefindlife,whetherinbubblinghotsprings,dropsofbrine

insideice,orfilmsofwateraroundmineralsdeepinthecrust.

Somegofurtherandspeculatethatweirdlife—organismsthatdon’tdependon

water—mightexistinlakesonTitan,thelargestmoonofSaturn.Titan’slakes

containliquidhydrocarbons,notwater,somewhatlikesmallseasofpetroleum.

Lifethatuseshydrocarbonsolventisunknown,andtherearesomearguments

fromphysicalchemistry,whichI’llmentionlater,thatsuchlifemightbe

difficult.Incontrast,there’snoquestionthatliquidwatercansupportlife.

Table1.NineabodeswherelifemightexistintheSolarSystemtoday.The

distancetotheSunisinAstronomicalUnits(AU),where1AUistheEarth–

Sundistanceofabout150millionkilometresor93millionmiles

Body Typeofbodyandaverage

distancefromtheSun

Whyitmighthave

life

Mars planet,1.5AU mighthavesubsurface

pocketsofliquidwater

Ceres largestasteroid,2.8AU mighthavea

subsurfaceocean

Europa,Ganymede,

Callisto

largeicymoonsofJupiter,5.2

evidencefor

subsurfaceoceans

Enceladus icymoonofSaturn,9.8AU evidencefora

subsurfaceoceanorsea

andpresenceof

organics

Titan largestmoonofSaturn,9.8AU evidencefora

subsurfaceoceanand

presenceoforganics

Triton largestmoonofNeptune,30.1

mighthavea

subsurfaceocean

Pluto largeKuiperBeltobject,39.3

mighthavea

subsurfaceocean

Sunlightandthehabitabilityoftheinnerplanets

Whethertheinnerplanets—Mercury,Venus,Earth,orMars—haveliquidwater

mostlyhastodowithtemperatureand,thus,distancefromtheSun(Table2).

‘Location,location,location’doesn’tjustsellhousesbutiscriticalforplanetary

habitability.

Table2.Theinnerplanetsandfactorsthataffecttheircurrenthabitability

Locationmattersbecausesunlightspreadsintoaspherewithasurfaceareathat

growswiththesquareoftheplanet–Sundistance.Solarfluxisthewattage(like

alightbulb)ofsunlightreceivedpersquaremetre.AttheEarth’sorbitaldistance

of1AstronomicalUnit(AU),sunshineprovides1,366Wattsovereverysquare

metre,theequivalentofalmostfourteen100-Wattlightbulbs.Atthe5AU

distanceofJupiter,thesolarfluxisafactorof25timessmallerbecausethesame

energyspreadsoveraspherethathas5×5=25timesthearea.ForMars,at1.5

AU,thesolarfluxis2.25(=1.5×1.5)timessmallerthanforEarth.Incontrast,

Venus,at0.72AU,receivessolarfluxalmosttwiceasbigasthatoftheEarth.

Mercury,atonly0.4AUfromtheSun,islifeless.It’sthesmallestoftheeight

planets(abouttwo-fifthsthediameterofEarth)andhasnoliquidwaterand

probablyneverdid.Today,Mercury’sbarrensurfacesometimesreaches430°C.

IfMercuryoncehadanatmosphere,itwouldhaveburnedoffwhenMercury

formedbecauseoflowgravityandintensesunlight.

ThedistancefromtheSunexplainswhyVenusandMarsarehostiletolife,

althoughthat’snotthewholestory.Theroughly460°CsurfaceofVenusiseven

hotterthanMercurygets,whileMarsisanicydesertwithanaverage

temperatureof–56°C.VenusiscomparableinsizetotheEarth,butMarsis

smaller—aroundhalfthediameterandone-ninththemassofEarth.Infact,

Mars’ssmallsizeledtoitspoorhabitabilitytoday,aswe’llseelater.

Althoughsolarfluxisonefactor,thegreenhouseeffectonMarsandVenusalso

determinessurfacetemperatures.TheMartianatmosphereexertsonly0.006bar

surfacepressure,comparedtoEarth’s1bar.Thiswispyairisanaridmixof95.3

percentcarbondioxide,2.7percentnitrogen,andminorgases.Recallthat

Earth’sgreenhouseeffectis33°C.Becauseofatmosphericthinnessanddryness,

Mars’sgreenhouseeffectisonly7°C,whichleavestheplanetfrozen.Thetwo

maingasesinVenus’satmosphere,96.5percentcarbondioxideand3.5percent

nitrogen,havesimilarproportionstothoseonMars.Butinstarkcontrast,

Venus’satmosphereismassivelythickandhasahugepressureof93baratan

averagegroundelevation.Asaresult,Venus’sgreenhouseeffectisawhopping

507°C,i.e.500°CbiggerthanonMars(Table2).Theupshotisthatneither

planet’ssurfacesupportsliquidwater.InthethinaironMars,apuddlewould

boil(orrapidlyevaporate)andfreezeatthesametime,andwatergenerally

existsonlyasiceorvapour.MeanwhileonthescorchingsurfaceofVenus,liquid

waterisimpossible.

Withoutliquidwater,Venusisconsideredbymostastrobiologiststobelifeless.

Afewspeculatethatacidophilemicrobesmightliveinitscloudsofsulphuric

acidparticles.However,Idoubtit.Apartfromthelackofavailablewater,

atmosphericturbulencewouldpullmicrobesdownintoVenus’sinfernoorupto

fatallydesiccatingheights.

WasVenusinhabitedinthepast?

Venusisstillastrobiologicallyinterestingbecauseitmayoncehavehadoceans

andlife.Itshouldhavebegunwithplentyofwaterbecausetheamountsofother

volatilesaresimilartoEarth’s.(Volatilesaresubstancesthatcanbecomegasesat

prevailingplanetarytemperatures.)Forexample,ifyoutookalloftheEarth’s

carbonaterocksandturnedthemintocarbondioxide(CO

),theEarthwould

haveaboutninetyatmospheres’worthofCO

,likeVenus.Ifyouextractedall

thenitrogeninmineralsontheEarthandaddedittotheEarth’satmosphere,you

wouldgetalmostthreeatmospheres’worthofnitrogengas,whichagainis

similartothatinVenus’satmosphere.Becauseaccretionofhydratedasteroids

wastheultimatesourceofcarbonandnitrogenonVenusandEarth,it’s

reasonabletoinferthatVenusalsogainedlotsofwaterfromthem,astheEarth

did.

UnfortunatelyVenuswasdoomedbyitsproximitytotheSunbecauseofa

runawaygreenhouseeffect.Whenbakedbyintensesunlight,theevaporationof

watercanmakeanatmospheresomoistthatitbecomescompletelyopaqueto

theinfraredradiationemittedfromtheplanet’ssurface.Atthispoint,there’sa

limittothecoolingoftheplanetbyemissionofinfraredradiationtospace,

whichissetbythepropertiesofwaterandaplanet’sgravity.Recentcalculations

suggestthatthisrunawaylimitisabout282WattspersquaremetreforEarthand

afewWattslessforVenus.

Tounderstandtherunawaylimit,considerturningEarthintoVenus.As

mentionedearlier,thesolarfluxattheEarth’sorbitis1,366Wattspersquare

metrebuttheEarthreflects30percentofthisandsoabsorbsonly70percent.

Thenanadditionalreductionof50percentcomesfromhavingonlyhalfofthe

Earthindaylight,andafurther50percentdecreaseaccountsforglancing

sunbeamsonEarth’scurvedsurface.Puttingallthesefactorstogether,theEarth

absorbsanet(0.7×0.5×0.5)×1,366=240Wattspersquaremetreofsunlight.

Whenstable,theEarthemitsthesameamountofenergyintospaceinthe

infraredandsokeepsaconstantglobalaveragetemperature.Butimagineifthe

EarthweremovedtotheorbitofVenus,wheretheabsorbedsunlightpersquare

metrewoulddoublefrom240to480Watts.Theoceanwouldbecomelikeahot

bathandthesteamyatmospherewouldreachtherunawaylimitwhereonly282

Wattspersquaremetrecanradiateintospace.Withmoreenergypersquare

metrecomingin(480Watts)thangoingout(282Watts),theEarthwouldsimply

gethotterandhotter.Theentireoceanwouldevaporateandsurfacerockswould

melt.Atthatpoint,near-infraredlightfromasearingupperatmospherewould

shinethroughtospace,sothattheincomingsunlightandoutgoingradiation

wouldcomebackintobalanceandthesurfacetemperaturewouldplateauaround

1,200°C.ThisnastyprocessiswhatwethinkhappenedtoVenus.AnyVenusians

wouldhavebeentoast.

TherunawaywouldalsocauseVenus’soceantovanish.Intheupperrunaway

atmosphere,ultravioletlightwouldsplitwatervapourintohydrogenandoxygen.

Hydrogenissolightthatitwouldescapeintospace,draggingalongsome

oxygen,whileanyoxygenleftbehindwouldoxidizehotrocksbelow.Eventually

rockswouldsolidify,butbythistimetheatmospherewouldbeloadedwith

carbondioxideandnitrogenreleasedfromthehotsurface.Theendresultwould

betoday’shellishVenus.

Beforetherunaway,lifemighthavethrivedinVenus’soceans.Recallthatthe

earlySunwaslessluminous.So,afewhundredmillionyearsmighthavepassed

beforetherunawaycommenced.Unfortunately,itmightbetrickytoproveiflife

everexisted.Inthe1990s,radaronNASA’sMagellanspacecraftmappedthe

Venusiansurface.Oldersurfaceshavemorecraters,sothedensityofimpact

craterswasusedtoestimatetheageofthesurfaceasafairlyuniform600–800

Ma.Venus’ssurfaceappearstohavebeenrepavedbylavaallatonce.Apossible

reasonisthatwhereasEarth’sinternalheatisreleasedcontinuouslybycreating

newseafloor,onVenusinternalheatmightperiodicallybuildupuntiltheinterior

becomessohotthatlavaeruptseverywhere.Venusmightbehavelikethis

becauseunlikeEarthitdoesn’thavewatertolubricateplatetectonics.

Resurfacingwoulddestroyanyfossils.However,subtlegeochemicaltraces

mightremain.ToinvestigateifVenushadlife,thebestoptionwouldbeto

collectsamplesofVenusianrocksandreturnthemtoEarthinafuturespace

mission.

WateryMars:anabodeforlife?

ManyhaveimaginedMars,unlikeVenus,asapotentialabodeforlife.Before

theSpaceAge,basicparameterswereknown,suchasMars’s24.66hourday,a

yearof1.9Earthyears,andgravitythatis40percentofEarth’s.Notmuch

aboutthesurfacewascertainuntilthefirstsuccessfulspacemission,theflybyof

NASA’sMariner4in1965,revealedaheavilycrateredsurfaceliketheMoon.

Thisputadamperonhopesforlife.Then,intheearly1970s,NASA’sMariner9

orbiterphotographeddried-uprivervalleysandextinctvolcanoes,suggesting

thatMarshadoncebeenquiteEarth-likeafterall.Butsoonthependulumswung

backintheotherdirection.TheVikingmission,consistingoftwoidentical

orbitersandlanders,reachedMarsin1976,andfailedtofindlife,aswediscuss

later.

Afterahiatusinthe1980s,explorationwasrevived.MarsGlobalSurveyor,

whichorbitedfrom1997to2006,mappedMarsandimagedsedimentarylayers

thatimpliedmanygeologiccyclesoferosionanddeposition.Intheearly21st

century,theMarsOdysseyandMarsExpressorbiters,alongwithMars

ReconnaissanceOrbiter,discoveredareasofclaymineralsandsalts,whichmay

haveformedinliquidwater.TwinMarsExplorationRoverslandedin2004,and

foundfossilizedripplesfrompastliquidwaterandsedimentaryrocks,whilein

2008,NASA’sPhoenixLanderdugupsubsurfaceiceinapolarregionand

measuredsolublesaltsinthesoil.Finally,in2012,CuriosityRovertrundled

towardsa5-kilometre-highmountainofsedimentarybedswithina150-

kilometre-diametercraternamedGale.Itfoundmudstonesdepositedfromwater

thatcontaintheSPONCHelements(Chapter1)neededforlife.

Nowadays,astrobiologicalinterestinMarsconcernseitherbiologicaltraces

frombillionsofyearsagoormicrobial-likelifethatmiserablyendures

underground.Pre-SpaceAgehopesofanEarth-likeMarstodayhavebeen

replacedwiththerealityofacold,windswept,globaldesertwithduststorms,

dailydustdevils,andnorainfall.

Mars’scurrentsurfaceishostiletolifeforthreereasons.First,whileiceexists

forsure,noliquidwaterhasbeenunequivocallyidentified.Thepolarcapsare

waterice,toppedwithcarbondioxide‘dryice’thatgrowswhenabout30per

centoftheatmospherefreezesatthewinterpole.Also,abovemidlatitudes,ice-

cementedsoilorpermafrostliesjustbeneaththesurface.Inthetropics,

afternoontemperaturesinthetopcentimetreofsoilriseabovefreezing,but

there,iceturnstovapourbeforemeltingtemperaturesarereached.Asecond

problemisnoozonelayer,allowingharmfulultravioletsunlighttoreachthe

surface.Third,chemicalreactionsintheatmospheremakehydrogenperoxide—

thesamechemicalusedinhairbleach.Hydrogenperoxidemoleculessettleto

thesurface,wheretheycandestroyorganics.

Whilethesurfaceisunpromising,geothermalheatundergroundmightallow

liquidwaterandlifetoexist.Infact,from2004,reportsofatmosphericmethane

averagingtenpartsperbillionbyvolumeledtoexcitementthatsubterranean

methanogensmightbepresent.SunlightreflectedfromMarsgatheredby

telescopesandMarsExpressappearedtoshowabsorptionbyatmospheric

methane.However,themethanesignalwasbarelydistinguishableandsome

scepticalscientists(includingme)doubtedwhethermethanewasreallypresent.

Subsequently,theCuriosityRoverhasfailedtodetectmethanedowntolevelsof

onepartperbillion.

Indiscussingpastlife,astrobiologistsrefertotheMartiangeologicaltimescale,

whichisdividedintoaeonscalledthePre-Noachian(before4.1Ga),Noachian

(4.1toabout3.7Ga),Hesperian(3.7to3.0Ga),andAmazonian(since3.0Ga).

SurfacesonMarsareplacedintoeachaeonaccordingtoimpactcraters.Older

surfaceshaveaccumulatedbiggerandmorenumerouscraters.Infact,thedates

boundingeachaeonactuallycomefromtheMoon.Theagesofrocksbrought

backbytheApolloastronautsareknownfromradioisotopesandthesecorrelate

lunarcrateringdensitieswithtime.Astronomicalcalculationsthataccountfor

moreimpactorsonMarsthantheMoonallowthelunarcorrelationtobe

extendedtoMars.

EvidencethatliquidwaterusedtobepresentsuggeststhatMarswasoncemore

habitablethantoday.Imagesshowfluvial(stream-related)featuresinthe

landscape,includinggullies,dried-uprivervalleys,deltas,andenormous

channels.Also,thesoilandrockscontainmineralsthatforminthepresenceof

liquidwater.

Gulliesareincisionsoftenstohundredsofmetreslengthonthewallsofcraters

andmesasbetween30and70°latitudesinbothhemispheres.Becausegullies

lacksuperimposedcratersandsometimesflowoversanddunes,theymustbe

veryrecent.Initially,theywerethoughttoformwhenicemelted.However,

imagesshowthemformingwhencarbondioxidefrostvapourizes,presumably

releasingadryflowofsoilandrocks.

Farmoreancientfeaturesarevalleynetworks,whicharedried-upriver-like

depressionsthatspreadoutintree-likebrancheswithtributaries(Fig.8).Most

inciseheavilycrateredNoachianterrainandthey’re1–4kmwideand50–300m

indepth.Thedensityoftributariesisfarlessthanformostterrestrialrivers,but

sometimesenoughtoinferthedrainageofrainormeltwaters.Inothercases,

valleyswithstubbytributarieswereprobablyformedbysapping,when

underground(melt)watercausederosionandcollapseoftheoverlyingground.A

fewvalleysendindeltas.

TheNoachianlandscapeconsistsofcraterswithdegradedrimsandshallow

floors.Whatcausedthiserosionisunclear.Valleynetworksareincisedontop

andsoweren’tresponsible.OncetheHesperianstartedat3.7Ga,erosionrates

droppeddramaticallyandvalleynetworksbecamerare.

TowardstheendoftheHesperian(around3Ga),outflowchannelsappeared

(Fig.8).Channelsformfromfluidflowconfinedbetweenbanks,lacking

tributariesandemergingfromasinglesource,unlikerivervalleys.Theoutflow

channelsarehuge:10–400kmwidth,upto1,000kilometresorsolong,andup

toseveralkilometresdeep.Mostbegininchaoticterrainwheretheground

collapsed,sometimesincanyonsorchasmswheretherearemountain-sized

heapsofsulphatesalts,whichmightbeevaporationresiduesfromsaltywater.

8.a)ValleynetworksonMars.Ontheleftistheerodedrimof456-km-

diameterHuygensCrater.Theimageiscentredat14°S,61°E,north

upwards.Scalebar=20km;b)OutflowchannelRaviVallis(0.5°S,318°E)

about205kmlong

Theleadingexplanationforoutflowchannelsisthattheyformedfrom

floodwaterswhenundergroundicemeltedoraquifersburst.However,they

require10–100timesmoreflowthanthebest-knownterrestrialanalogue:flood-

carvedlandacrosseasternWashingtonState,USA,whichearlysettlerscalled

scablands.Thescablandsformedattheendofthelasticeagewhenice

dammingofalargelakeperiodicallyruptured.

It’snoteasytoexplainthewaterneededtoerodeMars’soutflowchannels.

Estimatessuggesttheequivalentofaglobaloceanseveralhundredmetresdeep,

whichisfarmorewaterthanexistsasicetoday.Sincethechannelsflowtothe

northernlowlands,somescientistsspeculatethatanoceanformedthere.In

contrast,aminoritythinksthattheoutflowchannelswerenotcarvedbywater

butbylavas.ChemistrysuggeststhatMartianlavasshouldhavebeenrunny,

turbulent,anderosive.

Apartfromancientvalleynetworksandoutflowchannels(ifwatereroded),

mineralsprovideotherevidenceforawetterearlyMars.Everyoneknowshow

letteringonoldgravestonesdisappearsbecauseofchemicalreactionswithwater.

Suchreactionschangeordissolvemineralsinchemicalweathering.Muchofthe

Martiansurface,likethatonVenusandtheEarth’sseafloor,ismadeofbasalt,a

dark-colouredigneousrockrichinironandmagnesiumsilicateminerals.When

basaltischemicallyweathered,alterationmineralsareproduced,suchasclays.

Sothepresenceofalterationmineralsmeansthatliquidwaterwaspresent,

sometimeswithaspecificpH.Forexample,alkalinewaterstendtoproduceclay

mineralsfrombasalt.

Hydrous(water-containing)alterationmineralshavebeenidentifiedfrom

analysisofinfraredradiationemittedandreflectedbyMars’ssurface.Butonly

about3percentofNoachiansurfaceshavehydrousclaysandcarbonates.

SulphatemineralsdominatelateNoachianorHesperianareas,whilereddish,dry

ironoxidesarecommononyoungerAmazoniansurfaces.Thispatternmight

implythreeenvironmentalepochs.Inthefirstepoch,alkalineorneutralpH

watersweatheredbasaltandmadeclays.Duringthesecond,sulphuricacidwas

derivedfromvolcanicsulphurgasesandmadethesulphates.Thethirdepoch

continuestodaywithacold,dryenvironmentandrust-colouredsurfaces.

OneofthetwinMarsExplorationRovers,namedOpportunity,actuallylanded

nearNoachiansulphates.Millimetre-sizedspheresoftheironoxidehematite

(Fe

)wereembeddedinsulphatelayers,likeblueberriesinamuffin.The

hematiteprecipitatedfrommineralscarriedinwaterpercolatingunderground

about3.7billionyearsago.Also,ankle-deepwaterappearstohavepondedon

thesurface,leavingbehindripplesinthesediment.TheotherRover,named

Spirit,foundevidenceofancienthotspringsontheoppositesideoftheplanet.

TheearlyatmosphereandclimateofMars

EvidenceofliquidwaterleavesuswonderingwhethertheclimateonearlyMars

waswarmandwet.Scientistsdisagreeandfallintotwocamps.Onegroup

arguesthatMarshadawarm,wetclimatefortensorhundredsofmillionsof

years.Theothermaintainsthattransientmeltingoficeinacoldclimatecould

accountforwhatisseen.

Thefirstscenarioisobviouslymorefavourableforlife.Butunfortunately,no

onehasyetexplainedhowearlyMarswaskeptwarmformillionsofyears.

Some3.7billionyearsago,theSunwas25percentfainter.Agreenhouseeffect

ofabout80°CwouldhavebeenneededtokeepearlyMarsjustabovefreezing,

comparedtoEarth’smodern33°Cgreenhouse.It’sgenerallythoughtthatany

atmospherichydrogenshouldhaveescapedintospacerapidlywhenMars

formed,leavingtheatmosphereoxidizedandfullofcarbondioxideand

nitrogen.Marscouldn’teverhavehadanextremelythickVenus-likeatmosphere

becausecarbondioxidewouldcondenseintoiceandformcloudsatMars’s

distancefromtheSun.Athickcarbondioxideatmosphereisalsogoodat

scatteringsunlightbacktospace,whichcoolsthesurface.Socarbondioxide

can’tprovideanearlywarmclimate.Analternativesuggestionisthatvolcanic

sulphurdioxidewasthekeygreenhousegas.However,sulphurdioxidedissolves

inrainandwouldbeflushedfromtheatmosphereifMarsbecamewet.Also,

atmosphericreactionsathighaltitudesmakeafinesuspensionofsulphate

particlesfromsulphurdioxide,whichreflectssunlightandcoolstheplanet.Such

coolinghappenedonEarthduring1991–3asaresultofvolcanicemissionsfrom

MtPinatubointhePhilippines.

Theothercampproposesmanymechanismsthatallowliquidwaterinacold

climate.Theynotethatimpactswouldhavevaporizediceintosteam,which,in

turn,wouldhaveproducedrainfallthaterodedrivervalleys.Also,erosionmight

havebeenproducedbylocalsnowmeltasaresponsetopastfortuitous

combinationsofMars’saxialtiltandorbitalshape.Thesecharacteristicsare

perturbedovertimebythegravitationalinfluenceofotherplanets.Thetiltofa

planet’saxiswithrespecttotheplanet’sorbitalplane,e.g.23.5°forEarth,causes

theseasonsbecauseonehemispheregetsmoresunlightatonepointintheorbit

thantheother.UnlikeEarth,Marshasnobigmoontostabilizeitsaxis(itstwo

tinymoonshavenegligibleeffect),soMars’stilthasvariedbetween0°and80°

overthelast4.5billionyears.Athightilts,summertimepolaricefacestheSun

andvapourizes.Aircurrentstransportthevapourtothecoldtropicswhereit

snows.Sunlightduringotherseasonsoratlowertiltscouldthenproduce

meltwatersandfluvialerosion.Moderatetiltsinthelastfewmillionyearsmight

explainrelictmidlatitudepatchesofdustthatwereonceice-cemented.Finally,

saltywateronMarscanremainliquidfarbelow0°C.OnEarth,wespread

sodiumchlorideonicyroadsinwinterbecauseitmeltsicedownto–21°C.

Anothersalt,perchlorate,whichwasdetectedinMartiansoilbythePhoenix

landerinmagnesiumorcalciumform,candepressthefreezingpointofwater

below–60°C.

WhateverthetruthaboutearlyMars,Mars’ssmallsizeultimatelyspoiledits

habitability.Becausesmallobjectscoolfasterthanlargeones,internalheatwas

lostrapidly,sothatwidespreadvolcanismceasedlongago.Withoutvolcanism,

Marscouldn’trecyclecarbondioxide,leavingthegastobeconvertedinto

carbonates,whicharepresentbutnotintheabundanceexpectedifathickcarbon

dioxideatmospherehadallbeentransformed.So,inaddition,partoftheearly

Martianatmospherewasprobablyblastedawaytospacebylargecometand

asteroidimpactsinso-calledimpacterosion.Theatmospherewasvulnerable

becauseofMars’slowgravity.Alongwithmoregradualescapeofgasestospace

later,Marswasultimatelyleftwithitsthinatmosphere.

LookingforlifeonMars

Westilldon’tknowifMarshaslifeoreverhadlife,despiteattemptstofindit.

TheVikinglanderstriedtodetectlifein1976,and,sincethe1990s,scientists

havelookedforsignsofpastlifeinMartianmeteorites.

EachVikinglanderhadthreeexperimentstodetectmetabolismandafourthto

findorganicmolecules.Thefirst,thecarbonassimilationexperiment,examined

ifMartianmicrobesobtainedcarbonfromair.Martiansoil(sometimesmixed

withwater)wasexposedtocarbondioxide(CO

)andcarbonmonoxide(CO)

gasesbroughtfromEarth,withcarbon-14,aradioactiveisotope,ineachgas.

Afterwards,thesoilwasfoundtohaveincorporatedcarbon-14.Whena‘control

experiment’wasdonewherethesoilwasfirststerilizedat160°C,thesoilstill

tookupcarbon-14,whichsuggestedthatinorganicchemistrywasresponsible,

notMartianmicrobes.Asecondtestwasthegasexchangeexperiment,which

monitoredMartiansoilandasolutionoforganicsbroughtfromEarthtoseeif

gasesweregeneratedfrommetabolism.Strangely,O

wasreleased,butitwas

alsoreleasedfromsterilizedsoil.Evidently,chemicalsinthesoildecomposed

intoO

withwaterorheat.Thethirdinvestigation,thelabelledrelease

experiment,addedorganicscontainingcarbon-14tosoil.IfMartians

metabolizedtheorganics,theywouldgiveoffcarbon-14-containingCO

Radioactivegaswasemitted,whilesterilizedsoilreleasednoradioactivegas.At

facevalue,thiswasapositivedetectionoflife.Butmostscientiststhinkthatsoil

oxidantsreactedwithorganicstomakeCO

andwereinactivatedbyheat.A

reasontopreferthisexplanationcamefromthefourthexperiment,inwhicha

sortofelectronicnose,calledagaschromatographmassspectrometer,

identifiedmoleculeswaftingoffheatedsoil.Itfoundnoorganicmaterialinthe

soiltoadetectionlimitofafewpartsperbillion.

Overall,themoralisthatbeforelookingforextraterrestrials,youneedto

understandtheinorganicchemistryoftheenvironmenttoavoidfalsepositives.

Infact,Harold‘Chuck’Klein(1921–2001),wholedtheVikingbiology

experiments,toldmethathewantedtodothiswhentheVikingmissionwas

conceived,butmanagersatNASAwhocontrolledfinancesinsteadinsistedon

lookingforlifedirectly.

WhiletheVikingresultswerebeingponderedinthe1980s,aremarkable

discoverywasmade:therewerealreadyrocksfromMarsonEarth—Martian

meteorites!ImpactsknockrocksoffMarsandsomeofthesefallontheEarth.In

fact,about50kglandseveryyear,mostlyintheocean.Gassealedwithinsome

meteoritesduringminormeltingassociatedwithimpactejectionprovesa

MartianoriginbecauseitmatchestheatmospheremeasuredbytheViking

landers.OtherMartianmeteoriteswithouttrappedgashaveatripleoxygen

isotope(

O,

O,

O)compositionintheirsilicatemineralsthatisuniquetoall

Martianmeteoritesandidentifiesthemlikeafingerprint.By2013,sixty-seven

Marsmeteoriteswereknownbutthelistkeepsgrowing.

In1996,possiblesignsofpastlifeinALH84001werereported.ThisMartian

meteoritewasthefirstone(the‘001’)collectedintheAllanHills,Antarctica

(the‘ALH’),ina1984expedition(the‘84’).ALH84001crystallizedasan

igneousrockonMarsat4.1Ga.Insidetherockaresomecarbonateglobules

about0.1mmacrossthatformedat4.0–3.9Ga,andwithinthemarefour

possibletracesoflife:allegedmicrofossils;carbonatessaidtobeprecipitatedby

microbes;tracesoforganicscalledpolycyclicaromatichydrocarbons(PAHs),

whicharemadeofhexagonalringsofcarbonatoms;andcrystalsofthemineral

magnetite(Fe

)saidtobesimilartothosewithincertainbacteria.OnEarth,

magnetotacticbacteriamakemagnetitecrystalsinsidetheircellswithshapesthat

evolutionhashonedintomagnets.Thebacteriausemagnetitecompassesto

movealongtheup–downcomponentoftheEarth’smagneticfieldinorderto

findaboundarybetweenaloweroxygen-poorzonebelowandanupper

oxygenatedzone,whichoptimizestheirmetabolism.

Insubsequentyears,researchhascastdoubtonallfourclaims.Thealleged

microfossilsarerod-shapedstructuresthatmerelylooklikemicrobes.Scientists

havesincefoundinorganicmineralsurfaceswithsimilarshapes.Also,the

structuresinALH84001areabouttentimessmallerthanterrestrialmicrobesand

probablybeyondtheminimumvolumeneededforessentialbiochemistry.An

evaporatingfluidcanproducethecarbonatesintheglobulesandsothere’slittle

reasontoinvokebiogenicsalts,proposedasthesecondfeature.Regardingthe

thirdlineofevidence,analysesshowedthatmostPAHsgotintothemeteorite

whileitwassittinginAntarcticafor13,000years.PAHsareubiquitous

atmosphericpollutantsproducedfromburntorganicmaterial.Meteoritesare

darkandabsorbsunlightwell,sotheycansitinpuddlesinAntarcticiceduring

summertime,allowingwaterandchemicalstoinfiltratecracks.Ifthereare

MartianPAHsintheinteriorofALH8401,thesecouldhavebeenmade

inorganically.Therockwasheatedbyimpactsthatoccurredbeforetheejection

impact.Heatingshouldhavereleasedcarbon-bearinggasfromthecarbonates,

whichcanreactwithwatertomakeorganicmatterwithoutlife.Thefourth

argumentconcernedmagnetite.Itturnsoutthatonlyatinyfractionofmagnetites

inALH84001havebiogenic-likeshapes.Somescientistsarguethatmagnetiteof

manyshapes,includingbacteria-likeones,formedduringheatshockspriorto

ejectionwhenironcarbonatedecomposedintomagnetite.

ThecontroversyaboutALH84001showshowdifficultitistoprovethat

microscopiclifeexistsinoldrocks.Butsincemostterrestrialrockslackfossils,

absenceoflifeinALH84001doesn’tmeanthatlifeonMarsdidn’texist.We

needtokeeplooking.

Ceres:aseriouscandidateforhabitability?

BeyondMarsandwithinJupiter’sorbitisanasteroidbeltofmillionsofsmall

rockybodies,includingthelargest,Ceres,some950kmindiameter,at2.8AU

fromtheSun.Ceres’ssurfacecontainsclaymineralsandwaterice.AsonMars,

clayssuggestpastliquidwater.AleadingmodelforCeres’sinternalstructureis

arockycorethat’ssurroundedbya100-km-thickshellofice.Justabovethe

core,theremaybeasubsurfaceocean.Thiscouldbeverysalty,allowingits

persistenceatsub-zerotemperatures,andperhapsitonceflowedtothesurface.

CouldlifeexistinhydrothermalventsatthebottomofCeres’socean?Ceresis

sosmallthatthere’sprobablylittleinternalheatavailabletoday,soanybiomass

wouldbeextremelymeagre.Butperhapsinthepast,Cereswasmorehabitable.

NASA’sDawnmissionwillarriveatCeresin2015.Spectraofinfraredemission

andreflectedsunlightshouldgiveusmoredetailaboutthesurfacecomposition

andhabitability.

TheicyGalileanmoonsofJupiter

BeyondtheasteroidbeltisJupiter,whichisprobablynotarealistictargetfor

astrobiologyforreasonsgiveninChapter5.ButJupiterhassixty-sevenmoons

andthreemightbehabitable.Galileodiscoveredthefourlargestmoonsin1610,

whichareIo,Europa,Ganymede,andCallisto,goingoutwardsfromJupiter

(Fig.9).IoandEuropaaresimilarinsizetotheMoon,whileGanymedeand

CallistohaveproportionscomparabletoMercury.Theouterthreesatelliteshave

rockyinteriorscoveredwithice,whileIo’sexteriorisjustrocky.Io,Europa,and

Ganymedeprobablyalsohaveironcores.

9.TheGalileanmoonsofJupiter:Io,Europa,Ganymede,andCallisto

IoisthemostvolcanicbodyintheSolarSystem,withvolcanoesspewing

sulphurdioxide,whichfreezesontoIo’ssurface.ButIoisdevoidofliquidwater

andsurelylifeless.IoissovolcanicbecausegravitationalforcesfromJupiter

varyasIogoesroundinanellipticalorbit,squeezingIolikeastressball.This

tidalheatingofIoiscausedbyfrictionwhensolidmaterialmovesupanddown,

similartothewaythatwatersloshesinEarth’soceanictides.Io’sorbitisforced

tobeellipticalbecauseofperiodicgravitationalproddingfromEuropaand

Ganymede.ThetimesthatGanymede,Europa,andIotaketomakeorbitshavea

ratioof4:2:1,whichcausesthemoonstolineupcyclicallyandnudgeeachother

gravitationally.ThisrelationshipisthecalledtheLaplaceresonance,after

Pierre-SimonLaplace,theFrenchmathematicianmentionedinChapter2.

Europaalsohasanorbitthat’sforcedtobeellipticalandsoithastidalheating

too.TheheatingissmallerthanthatofIobecauseEuropaisfurtherfromJupiter.

TheaveragetemperatureofEuropa’sicysurfaceis–155°C,buttidalheatcan

maintainliquidwaterdeepundertheice.Impactcrateringdensitiessuggesta

surfaceageofonly20–180Maconsistentwithslushfrombelowrepavingthe

surface.Imagesshowchaoticterrain,i.e.disruptedsurfaceswithblocksthat

haveshifted,whichsuggestthatsubsurfaceicemightconvect.Aglobalnetwork

ofstripesandridgesincludesbandswherethesurfacehasrippedapartandice

appearstohavebeenextruded.Magnesiumsulphatesaltispresentonthe

surface,perhapsoriginatingfromwaterswithin,whileunknownmaterialscause

reddishstreaks.

ThekeyevidenceforEuropa’ssubsurfaceoceancomesfrommagnetic

measurementsbyNASA’sGalileospacecraft,whichorbitedJupiterfrom1995

to2003.UnlikeEarthorJupiter,Europa’sinteriordoesn’tcreateitsown,

intrinsicmagneticfield.ButinpassingthroughJupiter’slargemagneticfield,

electricalcurrentsareinducedinsideEuropa.Inturn,thesecurrentsgeneratea

weakandvaryinginducedmagneticfield.Thestrengthofthisfieldrequiresthat

anelectricallyconductingfluidexistwithin200kmofEuropa’ssurface.The

mostlikelyexplanationisasaltyoceanuptotwicethevolumeofEarth’socean.

Thelargestcraterssuggestthattheicecoverabovetheoceanisatleast25km

thick.However,someterrainthatpokesupmightbeproducedbyupwellingice

thatmeltsafewkilometresbelowthesurfaceandthenrefreezes.Soshallow

lens-shapedlakesmightexist.

Besidesliquidwater,thepossibilityoflifedependsonenergysources,

interfaces,andtheavailabilityofSPONCHelements.Europaprobablyhasa

goodsupplyofbiogenicelementsifitwasmadeofmaterialsimilartocarbon-

richmeteoritesorcomets.Moreover,heat-releasingreactionsofwaterandarock

seafloorcouldsupplyenergyaswellasnutrients.Radiogenicheat,whichis

producedbythedecayofradioactiveisotopeswithintherock(suchas

potassium,uranium,andthorium),mightproduceseafloorventsthatsupply

carbondioxideandhydrogenforchemoautotrophs.

Theavailabilityofoxygenintheoceanwouldalsopermitbiologicaloxidationof

ironandhydrogenontheseafloor.Therearetwosmallsourcesofoxygen.

ChargedparticlestrappedinJupiter’smagneticfieldslamintoEuropa’sicy

surfaceandbreakwatermolecules,releasingoxygen.Iftheicechurns,someof

theoxygencouldbecarrieddowntotheocean.Otheroxygencomesfromwater

moleculessplitbyradiationfromradioactiveelements.EstimatesofEuropa’s

biomassvaryfromaroundathousandtoabilliontimeslessthanonEarth.Some

optimistsevenspeculatethattheremightbeenoughoxygenforanimal-like

creatures!

MeasurementsofinducedmagneticfieldsonGanymedeandCallistoalsoimply

subsurfaceoceansbutdeeper,belowroughly200kmand300kmdepth,

respectively.Ganymede’saveragesurfaceageisabout0.5Ga,soitsoceanhas

probablynotgushedtothesurfacerecently.BecauseGanymedeisJupiter’s

largestmoon,itsinteriorsupportshigh-pressureformsoficethatwouldlie

beneaththeocean.Lackofarock–waterinterfacemightbelessfavourablefor

lifethanonEuropa.ThelowertidalheatingonGanymedealsoimpliesasmaller

biomass.

InferenceofasubsurfaceoceanonCallistoissurprisingbecauseCallistolacks

tidalheating.Thewarmthmustberadiogenicandtheoceanmightbeloaded

withsaltsthatlowerthefreezingpoint.Callisto’ssurfaceageis4Ga,sothe

prospectoffindingevidenceforoceaniclifetheremightbeslim.Also,withless

energyavailable,anybiomassshouldbeevensmallerthanthatonGanymede.

EnceladusandTitan:icymoonsofSaturn

Saturnanditssixty-twomoonslieatabouttwicethedistanceofJupiterfromthe

Sun.Thelargestmoons,discoveredbeforetheSpaceAge,are(goingoutwards):

Mimas,Enceladus,Tethys,Dione,Rhea,Titan,Hyperion,Iapetus,andPhoebe.

Enceladusisspecialbecauseithasactivegeology.It’sSaturn’ssixthlargest

moon,butits500kmdiametercouldalmostfitwithinGreatBritain.Enceladus

orbitsSaturntwiceforeveryorbitofDione.Asaresult,periodicgravitational

nudgesfromDioneforceEnceladus’sorbittobeelliptical,resultinginvarying

gravitationalforcesfromSaturnthatflexandheatEnceladus.Forreasonsthat

arenotfullyunderstood,theheatingisconcentratedunderEnceladus’ssouth

pole.There,icyparticlesandgassprayoutofparallelfracturesdubbedtiger

stripes,whichareabout130kmlong,2kmwide,andflankedbyridges.Thejets

containtracesofmethane,ammonia,andorganiccompounds,alongwithsalt.In

fact,thejetssupplyparticlestothe‘E-ring’ofSaturnwithinwhichEnceladus

orbits.

Enceladushasarockycore,anicyshell,andanundergroundseabeneaththe

areaofthejets.Thejackpotcombinationoforganicmolecules,energy,and

liquidwaterimpliesthatlifemightexistinsideEnceladus.Suchlifecouldbe

chemoautotrophic,feedingoffhydrogenproducedbywater–rockreactionsor

hydrogenandoxygenfromwaterthat’ssplitbyradioactivity.Iflifedoesexist

there,methaneororganicmatterinthejetscouldbebiological.

Titan,thelargestmoonofSaturn,isalsoofastrobiologicalinterest.It’ssimilarin

sizetoGanymedeandMercuryandistheonlysatelliteintheSolarSystemto

haveathickatmosphere.Infact,theairpressureatTitan’ssurfaceis1.5bar,

whichis50percentlargerthanthatattheEarth’ssurface.Theatmosphereof95

percentnitrogenand5percentmethaneprovidesa10°Cgreenhouseeffect,but

thesunlightatSaturn’sdistanceisonehundredtimeslessintensethanatEarth,

soTitan’ssurfaceisincrediblycoldat–179°C.

Titan’satmospherecontainsasmoggyhazeofhydrocarbons.Athighaltitude,

ultravioletsunlightbreaksupmethanemolecules(CH

)andsubsequent

reactionsbuilduphydrocarbonsincludingethane(C

),acetylene(C

propane(C

),benzene(C

),andreddish-brownparticlescontaining

polyaromatichydrocarbons.TheparticlesarecalledtholinsfromtheGreek

tholosfor‘muddy.’

Theproductsderivedfrommethanesedimentoutoftheatmosphere.Infact,

about20percentofTitan’ssurfaceiscoveredintropicaldunesthataremadeof

sand-sizedparticles,whichareatleastcoatedwithorganicsifnotmadeofthem.

Atthepoles,over400lakesaremixturesofliquidpropane,ethane,andmethane,

withextraordinarybeachesmadeofparticlesofbenzeneandacetylene.

MethaneonTitanbehaveslikewateronEarthandformsclouds,rain,andrivers.

MostofourknowledgeaboutTitancomesfromtheCassini–Huygensmission,

whicharrivedin2004.HuygenslandedonTitanin2005,whileCassiniisa

Saturnorbiter.NearthelandingsiteofHuygens,channelswereerodedbyliquid

hydrocarbons(Fig.10).Atthelandingsiteitself,cobbles(presumablymadeof

waterorcarbondioxideice)havebeenroundedasifbytransportinriversof

liquidmethane.Methanerainshouldbeveryinfrequent,butwhenitrains,it

pours.

Becausemethaneisdestroyedbysunlight,Titan’satmospherewouldrunoutof

methaneinabout30–100millionyearsifitwerenotresupplied.Howmethaneis

replenishedisamystery.Theleadinghypothesisisthatmethaneleaksoutof

Titan’sinterior,whichsuggestsgeologicalmovementinsideTitan.

Titanflexestoomuchtobeentirelysolid,whichisevidencethatTitanhasa

subsurfaceocean.TheeccentricityofTitan’sorbitensuresthatTitanissqueezed

bySaturn’sgravity.Theoceanexistsbelowanicycrustoflessthan100km

thickness.ButaswithGanymede,theseafloorshouldbedenseiceratherthan

rockbecauseTitan’smassallowssuchhigh-pressureformsofice.

10.a)Anetworkofchannelsthatappeartoflowintoaplainnearthe

Huygenslandingsite;b)ImageofthesurfaceattheHuygenslandingsite.

Stonesintheforegroundare10–15cminsizeandsitonadarker,fine-

grainedsubstrate

TwotypesoflifemightexistonTitan:Earth-likelifeinthesubsurfaceocean;

andweirdlifeinthehydrocarbonlakes.Regardingthefirstpossibility,when

Titanformed,theheatreleasedfromthecaptureofsmallerbodiesshouldhave

createdliquidwateronthesurfacetemporarily.Perhapswater-basedlifeevolved

andsurvivedasanoceanretreatedunderground.

Weirdlifewouldbelimitedinbiochemicalcomplexitybecauseoxygen,whichis

scarce,isrequiredforsugars,aminoacids,andnucleotides.Titancapturessome

oxygen-containingmoleculesfromspace,butthesupplyistiny.Hypothetical

lifeincoldhydrocarbonsolventsmightuseacetyleneforenergy.OnEarth,

acetyleneburnswithoxygeninweldingtorches.ButTitanianscouldmetabolize

acetylenewithhydrogenfromTitan’sair.Thechemicalreaction,

(acetylene)+3H

(hydrogen)=2CH

(methane)

producesenergy,buttheideaisspeculative.Themainproblemforweirdlifeon

Titan,Ithink,isthatliquidhydrocarbonsarepooratdissolvinglargemolecules,

whichareessentialforgenomes.Largermoleculesarelesssolublethansmaller

onesinliquidhydrocarbons.Furthermore,solubilitydeclinesatlower

temperatures.

SomeexoplanetsmighthaveTitan-likeliquidhydrocarbons.Themostcommon

starsarereddwarfs,whicharesmallerandcolderthantheSun.Exoplanetsnear

1AUdistancearoundsuchdwarfswouldhavesurfacetemperaturesintherange

ofliquidhydrocarbons,notliquidwater.Thus,ifweweretodiscoverweirdlife

onTitan,wemightimagineauniverseteemingwithlifeutterlydifferentfrom

ourown.Ironically,wewouldthenbetheweirdlife.

Triton:acapturedKuiperBeltobjectaround

Neptune

AtandbeyondtheorbitofNeptuneat30AU,objectsarecoveredinnitrogenice

).Triton,thelargestofthirteenmoonsofNeptune,andPlutoarebothreally

KuiperBeltobjects(KBOs).TheKuiperBeltisaregionoficybodieswithinthe

planeoftheSolarSystemat30–50AUleftoverfromSolarSystemformation.

TherearealsoKBOsscatteredoutto1,000AU.TritonwascapturedbyNeptune

becausetwofeaturesofitsorbitsuggestso:Tritonorbitsintheopposite

directiontoNeptune’srotation,anditsorbitalplaneistippedup157°with

respecttoNeptune’sequator.

Triton’ssurfaceistremendouslycold,around–235°C,becauseitreflects85per

centofsunlight.Anextremelythinnitrogenatmosphereexerts20millionthsofa

barsurfacepressure.This‘air’issimplythenitrogenvapourthatwillsitover

nitrogeniceatTriton’sprevailingtemperature.There’salsoalittlemethane

(about0.03percentofthenitrogenconcentration),whichisdestroyedby

ultravioletsunlight,creatingathinsmogofhydrocarbonparticles.Ifmethane

destructionhasbeenoperatingatthesameratefor4.5billionyears,abouta

metre’sdepthoforganicmaterialshouldhaveaccumulated.However,mobile

frostswouldcoverthisup.Areddishtinttosomeoftheicemightbethe

organics.Infact,sunlightsometimesvaporizesnitrogeniceintogeyser-like

plumes,whichcarrydarkparticlesthatareperhapsorganic.Apartfromnitrogen

ice,waterandcarbondioxideicemakeupsomeofthesurface.

Tritonisgeologicallyactivebecausecratercountssuggestthatresurfacingis

only10Maandthereareicystructureslikevents,fissures,andlavas.Wecall

thesecryovolcanic,meaningaformofvolcanisminwhichslushyicecomprises

theequivalentoflavaandmagma.Triton’sdensitysuggestsalargerockycore

thatsuppliessufficientradiogenicheatforcryovolcanismandpotentiallya

subsurfaceammonia-waterocean.

EarlyTritonmayhavebeenmorehabitable.Justaftercapture,Triton’sorbit

wouldhavebeenhighlyelliptical,producinghugetidalheatingfromNeptune

thatshouldhavemeltedanextensivesubsurfaceocean.Tidestendtocircularize

orbitsandtodayTriton’sorbitisclosetocircular.Thus,aftercapture,theice

shellgraduallythickenedastidalheatingdeclined.Today,theoceanmightbe

belowhundredsofkilometresofice,andwouldbeunderlainbyhigh-pressure

formsofice.Butalthoughoceaniclifemightbedeeplyhidden,thepresenceof

cryovolcanismsuggeststhatifwesampleorganicmatteronthesurface,there’sa

chanceoffindingtracesoflife.

DoesPlutohaveanunderworldocean?

Pluto,namedafterthegodoftheunderworld,hasasurfacemainlyofnitrogen

ice,andathinnitrogenatmosphereof8–15microbarsurfacepressure.There’s

alsoalittleatmosphericmethanethatsunlightdestroysjustasonTritonand

Titan.Again,thisproduceshydrocarbonparticles,whichsettleout,possibly

accountingfordarkareasonPluto’ssurface.

AcollisionbetweenPlutoandanotherKBOisbelievedtohavecreatedCharon,

thelargestofPluto’sfivemoons,inaneventanalogoustotheimpactthatformed

theEarth’sMoon.Thus,PlutoisreallyadoubleKBO:CharonishalfPluto’s

sizeandone-ninthitsmass.Every6.4daystheyorbitaroundapointthatlies

betweenthem.AftertheCharon-formingimpact,tidalheatingshouldhave

meltedasubsurfaceoceanonPluto.Plutoprobablyhasarockycorethatmight

supplyenoughradiogenicheattomaintainasubsurfaceoceanofammonia-rich

watertoday.

OrganismsinPluto’soceanwouldbelimitedbyenergy,sothebiomasswouldbe

farsmallerthanonEuropa.Anyoceanisalsoprobablyatdepthsexceeding350

km,anddifficulttoaccessinafuturespacemission,butlifemightbethere.

OurconsiderationofPlutoshowsthatlifemightexistinremarkablyunlikely

places,andmylistinTable1maybetooconservative.Manyofthecoldouter

SolarSystemobjectsusedtobedismissedaspotentialhabitats.Butperceptions

haveshifted.It’spossiblethattherearesubsurfaceammonia-richoceanson

Saturn’smoonRhea,Uranus’slargestmoonsTitaniaandOberon,andperhaps

somelargeKBOs,suchasSedna,whichissimilarinsizetoPlutoyet100AU

fromtheSun.GiventhatsomanyobjectsintheSolarSystemarepotential

refugesforlife,billionsofsimilarpossibilitiessurelyexistthroughoutour

galaxy.

Chapter7

Far-offworlds,distantsuns

Thehuntforexoplanets

BeyondtheSolarSystem,astronomershavediscoveredover3,400exoplanets,

includingcandidatesandconfirmedbodies.It’seasiertodetectlargeones,so

nearlyallarebiggerthanEarthandsomeareratherexotic.HotJupitersare

Jupiter-sizeplanetswithin0.5AUoftheirparentstars,someorbitinginonlya

fewdays.ThenthereareplanetsthatsoundlikeEarthonsteroids,theSuper-

Earths.Theseareuptotentimesthemassofourplanet.Althoughhardertofind,

it’sclearlyjustamatteroftimebeforemanyEarth-sizedexoplanetsbecome

known.Howmanywillbehabitableoreveninhabited?

Ofcourse,beforeidentifyingahabitableexoplanet,youhavetofindexoplanets.

Withthevastdistancesinvolved,thesearchisdifficultbutnonetheless

astronomershavedevelopedtwoclassesofmethods.Thefirst,indirect

detection,looksforstellarproperties,suchaspositionorbrightness,whichare

affectedbythepresenceofunseenplanets.Thesecondisdirectdetectionofa

planetwithanimageoraspectrumofitslight.

Thefourindirectdetectionmethodsare:astrometry,stellarDopplershift,

transits,andgravitationalmicrolensing.Thekeytothefirsttwoisthataplanet

andstarorbitacommoncentreofmass.Forexample,ifaplanetandstarhad

exactlythesamemass,theywouldorbitapointhalfwaybetweenthem.In

reality,aplanetissmallerthanastar,sothecentreofmassisclosertothestar

andperhapsinsideit.Butinallcases,thestarwillfollowalittleorbitaroundthe

centreofmassand‘wobble’evenwhenitsplanetscan’tbeseen.Inourown

SolarSystem,Jupiter’stwelve-yearorbitcausestwelve-yearwobblesofthe

Sun’sposition.Saturnaddsinanothersmallertwenty-nine-yearwobble,

correspondingtoitstwenty-nine-yearorbitaroundtheSun.

Astrometrymeasuresthemotionofastarintheskyusingtelescopes.It’s

sensitivetobigplanetsfarawayfromtheirstarsoitcouldbeusedtofind

planetarysystemssimilartoourown.Butyouhavetowaitmanyyearsor

decadestotracktheeffectsofplanetsfarfromtheirstar.

Thesecondtechnique,theDopplershiftorradialvelocitymethod,reliesonthe

factthatwhenthelightofastarissplitintoallitscolours,thespectrumhasdark

bandslikeabarcode.Theelementsinastellaratmosphereabsorbphotons

comingfromthestar’sinterior,whichcausethedarklines.Ifthestarmoves

towardorawayfromtheEarth,thelinesshifttohigher(bluer)orlower(redder)

frequencies,respectively.EveryonehasexperiencedtheDopplereffectinsound.

Whenawailingpolicecarapproaches,thesoundwavesarebunchedintoahigh-

frequencysqueal,butafterthecarpassesby,theybecomealow-frequency

drone.Asimilarfrequencyshiftoccurswithlight.Arhythmicredshiftandblue

shiftoflinesinastellarspectrumshowsthatthestariswobblingandhasa

planetaroundit.Thesizeoftheshiftindicatesthemassofaplanet,whilethe

pacinggivesthetimefortheplanettocompleteanorbit.

In1995,DidierQueloz(thenastudent)andhismentor,MichelMayor,fromthe

UniversityofGeneva,detectedthefirstplanetaroundaSun-likestarusing

Dopplershift.ItwasaplanetofatleasthalfaJupitermassorbitingastarinthe

constellationofPegasuseveryfourdays.Itwasatotalsurprise.Nooneexpected

agiantplanetsoclose(0.05AU)toitsparentstarbecausegiantplanets

shouldn’tformthere.Nowweunderstandthatsomeextrasolarplanetsundergo

planetarymigrationearlyintheirhistory(Chapter3)andweendupseeingwhat

survivedthejostling.In2012,DopplerdatasuggestedapossibleEarth-mass

planetjust4.3lightyearsaway,orbitingthestarAlpha-CentauriB.Theplanet,if

present,lies0.04AUfromAlpha-CentauriB,whichissoclosethatitssurfaceis

probablymoltenrock.

OnesubtletywiththeDopplershifttechniqueishowweviewtheplanetary

system.Iftheorbitisface-on(describedasaninclinationofzerodegreesfroma

planeperpendiculartothelineofsight),there’snoto-and-froDopplershift.The

moreedge-ontheorbit,thegreatertheDopplershift.Itbecomesmaximalatan

edge-oninclinationof90degrees.There’softennowayofknowingthe

inclination,soameasuredDopplershiftmightbesmallerthantheidealedge-on

case,allowingusonlytoinferaminimumplanetarymass.TheDoppler

techniqueismostsensitivetobigplanetsclosetotheirstar,sothat’swhymostof

theexoplanetsinitiallydetectedwerehotJupiters.Butwenowknowfromthe

transitmethodthathotJupitersareactuallyonlyatinyminorityofexoplanets,

around0.5to1percent.

Thetransitmethodmeasuresthedecreaseinstarlightwhenaplanetcrossesthe

faceofastar,whichcanhappenifwe’reluckyenoughtoviewanexoplanet

systemvirtuallyedge-on.Suchgeometryisstatisticallyrare,butthereareso

manystarsthatifyougazewidelyandlongenough,transitswillbeseen.From

thedipinstarlight,it’spossibletodetermineaplanet’sdiameterifthestar’ssize

canbeestimated.Inturn,theplanet’sorbitalperiodanddistancefromitsstar

canbecalculatedfromthecyclingofthedimming.NASA’sKeplermissionisa

telescopethatoperatedfrom2009to2013fromanorbitaroundtheSunstaring

continuouslyat145,000mainsequencestarsintheconstellationofCygnus(the

Swan)tolookfortransitswheremostofthestarsare500–3,000lightyears

away.Keplerhasfoundover3,200exoplanetcandidates,whichareconfirmed

bycheckingforperiodicdimmingorusinganotherdetectiontechnique,suchas

Dopplershift.In2017,theTransitingExoplanetSurveySatellite(TESS)

telescopewillbeginexaminingtwomillionstarsfortransits.

Thefourthindirectmethodismicrolensing.Whenanobjectpassesbetweena

distantstarandus,Einstein’sTheoryofGeneralRelativitypredictsthebending

oflightbytheobject’sgravitationalfield.Theforegroundobjecteffectivelyacts

asalens,focusingthelightandmakingthedistantstarappeargraduallybrighter.

Butifaplanetorbitsalensstar,thebackgroundstarcanbrightenmorethan

once.ThissensitivetechniquecanfindEarth-massplanetsatorbitaldistancesof

1.5–4AUaroundastar.Unfortunately,oncethealignmenthashappened,it’s

virtuallyimpossibletofollowupwithmoredetailedmeasurementsbecausethe

objectsaretypicallyverydistant.However,microlensingcangatherstatistics.

Largeplanetsdriftingaloneininterstellarspacecanalsoactaslensobjects,and

oneresultisthatthereappearstobemanyunboundJupiter-massplanetsfloating

betweenthestars.Theselonelyworldswerepresumablyejectedfromtheir

extrasolarsystems.

Forastrobiology,themostimportanttechniquesaredirectdetectionsthatcapture

lightfromaplanet.Directdetectionischallengingbecauseaplanetisadim

bodyclosetoavastlybrighterstar.Nonetheless,telescopesinspace,suchasthe

HubbleSpaceTelescope,andextremelylargetelescopesonthegroundhave

accomplisheddirectdetectionusingacoronograph,whichisamasktoblockthe

starlight.Theterm‘coronograph’comesfromtechniquesthatwereoriginally

usedtoblockoutsunlighttostudytheSun’scorona,whichisthewispyhalo

seeninasolareclipse.Ground-basedtelescopesalsouseadaptiveoptics,which

areprocedurestosenseandcorrectthedistortionscausedbytheshimmeringof

theEarth’satmosphere.

Aspace-basedtelescopehastheadvantageofavoidingtheblursfromtheEarth’s

atmosphere,butthere’sthequestionofwhichpartofthespectrumtoexamine.In

general,planetsemitmostlyinfraredradiationandreflectvisiblestarlight.Ifwe

lookedatourSolarSystemfromafar,theSun,beinghotandbig,wouldoutshine

theEarthbyafactorofabouttenmillionintheinfraredandtenbillioninvisible

light.SolookingforadistantEarthintheinfraredgivesathousandtimesbetter

contrastthaninvisiblelight.Unfortunately,lightalsospreadsandblurs

(diffracts)whenitencountersanyobjectsuchasatelescopeandthisisworse

withinfraredlightthanwithvisible.Fortunately,atechniquecalled

interferometrycancancelunwantedlight.Noise-cancellingheadphonesusethe

sameprincipletosilenceundesirablesoundwaves.

Lightwaveshavecrestsandtroughslikewaterwaves.Nullinginterferometry

usesmorethanonetelescopemirrortopreciselyalignlightwavesarrivingfrom

apointintheskysothatwavecrestscanceltroughsandlightisturnedinto

darkness.Inthisway,starlightcanbe‘nulled’toseeplanets.BothNASAandthe

EuropeanSpaceAgencyhavestudiedspacetelescopes,calledTerrestrialPlanet

FinderandDarwin,respectively,whichuseinterferometrytocaptureimages

andspectraofexoplanetsaroundnearbySun-likestars.

Theresultsofexoplanetsurveystodategenerallyshowthatthenumberof

exoplanetsincreasesatlowermasses,sorocky,Earth-sizedplanetsshouldbe

common.Densityestimatesforsomeexoplanetsalsoallowassessmentof

composition.DensityhasbeeninferredusingmassfromtheDopplermethodand

sizefromtransitmethods.Also,ifatransitingEarth-sizedplanethasatleastone

Neptune-likesibling,amethodcalledtransittimingvariationscanprovidethe

massofplanetsandthusdensity.Evenifthelargerplanetdoesn’ttransitits

parentstarbecauseofitsorbitalinclination,itcancauseslightvariationsinthe

smallerplanet’stransittimesthatallowplanetarymassestobecalculated.With

densitymeasurements,thereareincreasingcluesaboutwhichexoplanetsare

madeofgas,rock,water,orsomemixture.

Thehabitablezone

WhenwewereconsideringlifeintheSolarSystem,weconcentratedonbodies

withliquidwaterbecausewe’resurethat’sarequirementforEarth-likelife,and

thesameideaappliestoexoplanets.Whereasdryplanetswithsubsurfaceoceans

areworthinvestigatingwithspaceprobeswithinourSolarSystem,they

generallywouldn’tbetargetsforexoplanetastrobiology.There’snoprospectin

thenearfutureofsendingspacecrafttovisitexoplanets,soeverythingmustbe

donewithtelescopeslookingatlightfromafar.Consequently,wecaremost

aboutexoplanetswithliquidwaterrightatthesurface.Thesehavethechanceof

abiospherethatpumpslotsofgasesintoanatmosphere.Inprinciple,such

biogenicgasesaredetectableinthelightfromtheplanetandmightindicatelife.

In1853,WilliamWhewellnotedthatEarth’sdistancefromtheSunallowed

liquidwaterbetweenwhathecalleda‘centraltorridzone’andan‘externalfrigid

zone’.Inthe1950s,theAmericanastronomerHarlowShapley(1885–1972)

(whodiscoveredthedimensionsofourgalaxy)alsotalkedaboutazonearound

starswhereplanetscouldhaveliquidwaterontheirsurface.Ifaplanetistoofar

awayfromitshoststariticesover,andifit’stoocloseit’stoohotforliquid

water.Nowadays,thetermhabitablezone(HZ)referstotheregionaroundastar

inwhichanEarth-likeplanetcouldmaintainliquidwateronitssurfaceatsome

instantintime.Wespecifyaparticulartimebecausestarsageandbrightenor

dim,sotheHZmoves.IncontrasttotheHZ,thecontinuouslyhabitablezone

(CHZ)istheregionaroundastarinwhichaplanetcouldremainhabitablefor

somespecifiedperiod,usuallythestar’smainsequencelifetime.

ThewidthoftheHZarounddifferenttypesofmainsequencestars,includingthe

Sun,hasbeenestimated.Theinneredgeissetbyaplanet’ssusceptibilitytothe

runawaygreenhouseeffect,whiletheouterboundaryisusuallydeterminedby

thefailureofgreenhousewarmingwhencarbondioxideiscoldenoughto

condenseintocloudsofdryiceordenseenoughtoscatterawaysunlight.The

latterwasapotentialproblemforearlyMars(Chapter6).FortheSun,theHZ’s

inneredgeisaround0.85–0.97AU,whiletheperipheryis1.4–1.7AU.The

spreadreflectstheuncertaineffectsofwaterandcarbondioxidecloudsatthe

innerandouterborders,respectively.Forexample,cloudsmightcoolaclose-in

planetbyreflectingsunlight,sotheinneredgemightbe0.85insteadof0.97AU.

Forthemostoptimisticouterboundary,MarsresideswithintheHZ.However,

Mars’ssmallsizehasledtoathinatmospherethatcan’tsustainliquidwater.In

otherwords,ifMarshadbeenasbigastheEarth,perhapsitmightbehabitable

today.SoplanetsizemattersandtheHZisnotthewholestoryforhabitability

butjustafirstguide.

Aroundotherstars,theHZiscloserinorfurtheroutdependingonwhetherastar

iscoolerorhotterthantheSun.Forexample,aKstar,whichiscooler(Fig.1),

wouldhaveatighthabitablezonefromabouttheorbitofMercurytothatofthe

Earth.AnevencoolerM-typereddwarfwouldhavethecentreofitshabitable

zoneatonly0.1AUorso,wellwithinthe0.4AUorbitofMercury.

Infact,theHZoffaintMdwarfsissosnugthatplanetswillexperiencetidal

locking,whichoccurswhengravitysetstherotationperiod.Thesame

phenomenonmakesonefaceoftheMoonpointtowardtheEarth.TheMoonis

tidallylockedandspinsonitsaxisonceforeveryorbitaroundtheEarth.The

matchbetweenorbitalandrotationperiods,calledsynchronousrotation,isno

coincidence.TheMoonelongatesslightlyintheEarth–Moondirection,andif

thisorientationdeviated,theEarth’sgravitywouldtwisttheMoon’sskewed

bulgesbackintoalignment.

AsynchronouslyrotatingplanetintheHZofanMdwarfwouldhaveonesidein

sunlightandtheotherinperpetualdarkness.However,thisdoesn’tmakesuch

planetsuninhabitable.Althoughthenightsidewouldbecold,amoderatelythick

atmospherecantransportenoughheattowarmthenightsidewhilecloudcover

cancoolthedayside.Also,alargemoonorbitingsuchaplanetwouldhave

variablesunlight.SotidallockinginthehabitablezonesofMdwarfsisnota

showstopperforlife.That’sencouragingbecauseMdwarfsarethemost

commontypeofstars,makingupaboutthree-quartersofthetotal.

Inrecentyears,assumptionsdefiningtheHZhavebeenquestioned.

Conventionally,theouteredgeisdeterminedbycarbondioxidecondensation.

Butsomelarge,rockyexoplanetsmighthavethickhydrogen-richatmospheres.

WeknowfromstudyingTitanandthegiantplanetsthathydrogenandmethane

behaveasgreenhousegases,andthesecondenseatfarcoldertemperaturesthan

carbondioxide.Also,ontheinneredge,awater-poorplanetwithpolarlakes

ratherthanoceansmightnotsuccumbtoarunawaygreenhousebecauseit

wouldn’thaveenoughwatertogenerateanatmospherecompletelyopaqueto

infraredradiation.Thus,theHZmightbewiderthanwethink.

Isthereagalactichabitablezone?

Someastronomersalsoarguethatthere’sanoptimalregioninthegalaxyfor

habitableplanetarysystemscalledthegalactichabitablezone(GHZ).Theynote

thattheSunistwo-thirdsofthewayfromthecentreoftheMilkyWay,whereas

planetarysystemsnearthedenselypopulatedcentrewouldbeperturbedby

supernovaeorpassingstars.Attheotherextreme,starsnearthegalaxy’sedge

areverypoorinelementsotherthanhydrogenorhelium,andthismightcurtail

planetformation.Astronomershaveanoddconvention(whichsickens

chemists!)ofcallingallelementsotherthanhydrogenorhelium‘metals’;the

‘metal’contentofastariscalleditsmetallicity.Starswithgiantexoplanetstend

tobemetalrich,andsincegiantexoplanetsweretheonesdiscoveredfirst,itwas

initiallythoughtthathighmetallicitywasneededforplanetstoforminanebula.

Morerecently,nocorrelationwithmetallicityhasbeenfoundinstarsofSun-like

massthathaveSuper-Earths.

AnotherproblemwiththeGHZisthatstarsdon’tstayput.Recentresearch

showsthatstarswanderacrossthegalacticdiscasaresultofgravitational

scatteringbyspiralarms.Inanycase,theexoplanetsthatwillbescrutinizedfor

signsoflifeintheforeseeablefuturewillallbeclose,withinahundredlight

yearsofus.SowhateverthevalidityoftheGHZ,it’snotapractical

considerationforfindinghabitableplanetsanytimesoon.

Biosignatures,orhowwefindinhabitedplanets

Ofcourse,thecentralquestioniswhetherwecanfindlife.In1990,NASA’s

Voyager1spacecraftwasheadingoutoftheSolarSystemandlookedbackatthe

Earthfrom6.1billionkmaway.Thefamouspictureittookisknownasthe‘Pale

BlueDot’,inwhichEarthissosmallthatit’sasinglebluishpixel.Ifweknew

nothingelse,whatcouldwededuce?Ifwesaidthattheplanet’scolourwasa

mixtureofblueoceansandwhitecloudsthatwouldjustbeguesswork.Howcan

wedobetter?

Generally,todetermineanexoplanet’spropertiesandlookforlife,weneeda

planet’svisibleorinfraredlightspectrum.Differentmoleculesandatomsabsorb

differentfrequenciesinspectra.Thus,wecanlookforatmosphericgasessuchas

oxygenormethanethatcanbeproducedbylife.Suchplanetaryfeaturesthat

indicatelifearebiosignatures.Infact,inthesameyearasVoyager1’s‘PaleBlue

Dot’,theGalileospacecraft,whichwasheadedforJupiter,obtainedspectraof

Earth.Itwasasortofdryrunforexoplanetsandshowedthatyoucoulddetect

Earth’satmosphericoxygen,methane,andabundantwater.Thesimultaneous

presenceofoxygenandmethaneisevidenceforlifebecausethesetwogases

shouldquicklyreactwitheachotherunlessabiosphereisproducinggreat

amountsofthem,whichpreventsequilibrium.Thus,wesaythatEarth’s

atmosphereisinchemicaldisequilibrium.That’sthekindofbiosignaturewe

wouldlikefromexoplanets.

Already,someinformationhasbeencollectedaboutexoplanetatmospheresby

lookingatdifferencesinspectrawhentransitingexoplanetspassbehindorin

frontofastar.Thespectrumintheprimarytransitwhenaplanetcrossestheface

ofastarincludeslightpassingthrougharingofatmospherearoundtheplanet.

Sosubtractingthespectrumofjustthestaraftertheplanethaspassedbycan

isolatethespectrumoftheplanet’satmosphere.Alternatively,youtakea

spectrumwhentheplanetisbehindthestarinitssecondarytransitandyou

subtractthatfromaspectrumoftheplanetplusstarwhentheplanetisbesidethe

star.Thisproducesthespectrumofthewholeplanet.Infact,thistechniquehas

beenusedtoobtaininfraredspectraofhotJupiterswiththeSpitzerSpace

Telescope,whichisatelescopeorbitingtheSun.

Withmoresophisticatedtelescopes,wewouldliketodetermineanEarth-like

exoplanet’ssurfacetemperatureandwhetherithasliquidwater.Aspectrum

mightprovidethat,butyoumightonlybeabletoseelightfromcloudtopsorthe

upperatmosphere.Infact,Venusisveiledinthisway.Forexoplanetsthatare

similarlyobscured,we’llneedtoinfertheamountofgreenhousegasesfrom

absorptionlinesinspectraandcalculateasurfacetemperature.Togetherwithan

exoplanet’satmosphericcomposition,youcouldtheninferwhetherliquidwater

ispresent.Itmightalsobepossibletolookforanoceanthroughglint,whichis

thebrightreflectionspotonasmoothbodyofwateratglancingangles.Finally,

weshouldkeepanopenmindaboutanti-biosignatures.Microbiallifewill

readilyeathydrogenorcarbonmonoxidegas,soanabundanceofeitherofthese

mightbeconsideredananti-biosignatureforaplanetinthehabitablezone.

Basically,anti-biosignaturessay‘noonehome’.

Thesearchforextraterrestrialintelligence(SETI)

Adifferentapproachtofindinglifeonexoplanetsistohypothesizethat

technologicalcivilizationsexist.Iftheydo,wecanlookfortheir

communications.Thisendeavouristhesearchforextraterrestrialintelligence

(SETI)ormoregenerallythesearchfortechnosignatures.

In1959,GuiseppeCocconiandPhilipMorrisonadvocatedlookingfor

broadcastsofextraterrestrialcivilizationsusinglargeradiotelescopes.Others

havesincesuggestedwatchingfortransmissionsinvisiblelight.Thereare

practicalreasonstolookonlyforthesesignals.Forexample,X-raysare

absorbedintheupperatmosphere,whereasradioandvisiblelightareeasyto

generateanddetectableacrossspace.ThemainquestionforSETIiswhetherit’s

worthlooking.Arethereenoughcivilizationsoutthere?

In1961,FrankDrake(thenatCornellUniversity)describedawaytoevaluate

thepotentialnumberoftransmittingcivilizationsinourgalaxy.Ifweestimate

theaveragenumberofcivilizationsthatcome‘onair’eachyearandtheir

averagelifetime,wecanmultiplythosetwoquantitiestogethertogivethe

currentnumberoftransmittingcivilizations.Togiveananalogy,ifthenumber

ofnewfreshmenstartingatauniversityisaround6,000peryearandtheyspend

anaverageoffouryearsoncampus,thetotalundergraduatepopulationatany

onetimeis6,000×4=24,000.Drakewentfurtherbydevisingsixfactorsthat

determinethenumberof‘freshman’transmittingcivilizationsappearingper

year.Infull,wecalculateN,thenumberofcommunicatingcivilizations,by

multiplyingthesixnumbersandtheaveragelifetime,asfollows:

Ncivilizations=

ThisisthefamousDrakeEquation.ThefirstnumberR,isthebirthrateofstars

suitableforhostinglife.Astronomersobserveabouttennewstarsperyearof

typesG,K,andM.Alltheotherfactorsareasfollows:

planet

isthefractionofsuchstarshavingplanets;

habitable

istheaveragenumberofplanetsperplanetarysystemthatarehabitable;

life

isthefractionofthoseplanetsonwhichlifeoriginatedandevolved;

intelligence

isthefractionofinhabitedworldsthatdevelopedintelligentlife;

civilizations

isthefractionofthoseworldsthatdevelopedcivilizationscapableofinterstellar

communication;

andListhelifetimeofthosecommunicatingcivilizations.

Severalfactorsintheequationareunknown.But,whattheheck?Let’susethe

DrakeEquationanyway.Currentexoplanetsearchessuggestthatatleasttwo-

thirdsofallstarshaveplanets,solet’stakef

Planet

as2/3.DatafromtheKepler

missionarestillbeinganalysed,butsuggestatleastthat1in100ofplanetsare

habitable,solet’ssetn

habitable

=1/100.Wewillsimplyhavetoguessallother

parameters.LifedevelopedrapidlyonEarth,solet’spresumethatlifeoriginates

onhalfofthehabitableplanets,i.e.f

life

=1/2.Let’ssupposethatthefractionof

biospheresthatdevelopintelligenceisf

intelligence

=1/8.Let’salsoguessthatone

intenintelligentbiospheresdevelopcivilizationscapableofinterstellar

transmissions,sof

civilizations

is1/10.Finally,thelifetime,L,ofcommunicating

civilizationsissociologicalspeculation,butlet’ssay10,000years.Sohow

manycommunicatingcivilizationsintheMilkyWaydoweget?Theansweris

four,from10×(2/3)×(1/100)×(1/2)×(1/8)×(1/10)×10,000.Fine.Butcan

weconstraintheprobabilitiesbetter?

Exoplanetstudieswilleventuallynaildownthesecondandthirdtermsinthe

DrakeEquationandmighthavesomethingtosayabouttheprobabilityoflife

arising,f

life

,ifbiosignaturesaredetected.Optimiststhinkthataplanetwith

liquidwaterandtherightmaterialsdevelopslifeeasily.Atpresent,wedon’t

reallyknow.

Thenexttermconcernsintelligence,sohowprobableisthat?Arelevantissue

mightbethatonlysomebiologicalsolutionsworktosolvespecificproblems.In

zoology,weoftenfindthesametraitsharedbyorganismsinunrelatedlineages,

aresultofwhatiscalledconvergentevolution.Convergenceoccursforspecific

functionsandecologicalniches.Forseeing,eyeshaveevolvedinatleastforty

differentanimalgroups.Adog’slife,ofallthings,isanexampleofaconvergent

niche.TheTasmanianwolf(amarsupial)evolveddog-liketraitssimilartothe

Mexicanwolf(aplacentalmammal).Evolutionaryconvergenceissocommon

thatitmeansthatorganismswithlegs,pairsofeyes,andcertainecologiesmight

beinevitablebecauseonlythesearephysicalsolutionstotheproblemsof

walking,stereovision,andoccupyingparticularniches.

Whethertechnologicalintelligenceisuniqueorconvergentiscontentious.On

Earth,itwasslowtoappear.IttookfourbillionyearstobuildupsufficientO

to

allowanimallife.Thenanotherfewhundredmillionyearspassedbefore

technologicallife.Dinosaursreignedforabout170millionyearsandyetwefind

nosignoftechnology,neitherfossiltoolsnordinosaurmicrowaveovens.Onthe

otherhand,therearesomelineageswherebrainsappeartohavehadvalueand

grown.Brainmassitselfisapoormeasureofintelligencebecauseabigbody

oftenneedsabigbraintorunit.Instead,theEncephalizationQuotient(EQ)is

used,whichistheratioofthebrainmassofananimaltothebrainmassofan

averageanimalofthesamebodymass.Thus,anaverageanimalhasanEQof1.

Scoreshigherthan1arebrainyandscoresbelow1morebrainless(Fig.11).

HumansandchimpshaveEQsofabout7.4and2.5,respectively,whichmeans

thattheyhavelargerbrainsthanexpectedbythesefactors.ArabbithasanEQof

0.4,whichwouldpleaseElmerFudd.AsAristotlenoted,‘manhasthelargest

braininproportiontohissize’.

11.Brainandbodymassforsomedifferentmammals.Animalsthatplotto

theleftofthediagonallinehavemorebrainmassthantypical

PalaeontologistshavediscoveredanincreaseinEQincertainlineagesovertime,

mostnotablythegenusHomo.TheEQofHomohabilis,ahomininthatlived

about2Ma,wasonly4,forexample.Intoothedwhales(dolphins,sperm

whales,andorcas),EQincreasedaround35Ma,whentheydeveloped

echolocationtofindfishandfriends.‘Friends’maybethekey.Generally,

intelligenceislargerinsocialanimals,suchas5.3foradolphin.Intelligence

probablyaidssurvivalandgettingamate,which,inturn,promotesmore

offspring.However,there’smuchdebateaboutwhichevolutionarypressuresare

behindintelligence.

AfinalquestionaboutSETIiscalledtheFermiParadox,andwasconceivedby

EnricoFermi(1901–54),theNobelPrize-winningphysicistwhobuiltthe

world’sfirstnuclearreactorin1942.Hisideaarosefromalunchtime

conversation.StarsintheMilkyWaydiscdatefromninebillionyearsago,

whereastheEarthisonly4.5billionyearsold.Thus,ifintelligentlifeis

common,manytechnologicalcivilizationsshouldhavearisenlongbeforeus.

Assumingthattheydevelopedspacetravel(orself-replicatingrobotstodothe

spacetravelforthem),theyshouldhavespreadthroughoutthegalaxybynow.

‘Wherearethey?’Fermiasked.

Fermi’sParadoxhasthreesolutions.Oneisthatwe’realoneintheMilkyWay

becauseonefactorintheDrakeEquationisvanishinglysmall.Asecondisthat

Fermi’spremiseisincorrect.Forallmannerofreasons,civilizationsmightnot

goaroundcolonizingthegalaxy.Finally,therearecivilizationsbutthey’re

hidingtheirexistencefromus(ormostofus).Scientistsparticularlydislikethe

thirdoptionbecauseit’sanadhochypothesisthat’suntestable,butit’sbeloved

ofsciencefictionwriters,tabloidnewspapers,and(accordingtopolls)one-third

ofAmericanadultswhobelievethatextraterrestrialshavevisited.The

reasonableoptionsarethefirsttwo.Obviously,alleffortstofindlifeelsewhere

bearuponthequestionofwhetherwe’realoneandthefirstsolution.IfSETI

succeeds,wemightalsogaininsightintoothercivilizations,ifwecould

understandtheirsignals.Atthemoment,theonlyknownlifeishere,so,like

Fermi,wecanonlywonder,‘Wherearethey?’

Chapter8

Controversiesandprospects

TheRareEarthHypothesis

Thebig,unansweredquestionsofastrobiologygeneratecontroversy.Onedebate

surfacedin2000whenPeterWardandDonBrownlee,mycolleaguesatthe

UniversityofWashingtoninSeattle,publishedabestsellingbook,RareEarth.In

essence,theirRareEarthHypothesisisthatthefortuitouscircumstancesthat

haveallowedcomplexlifeontheEartharesouncommonthatEarthmight

harbourtheonlyintelligentlifeintheMilkyWay.Amongsttheirarguments

werethegoodfortuneofEarthbeingintherightplaceinthegalaxy,having

JupiterinourSolarSystemtocapturecometsthatmightotherwisecollidewith

theEarth,Earth’sunusualrecyclingofvolatilesbyplatetectonicstokeepthe

atmospheregoing,thecontingenciesinobtaininganoxygen-richatmosphere,

andtheluckofhavingalargeMoonthatstabilizestheEarth’saxialtiltandsoits

climate.

RareEarthwasapolemicthatrailedagainsttheCopernicanPrinciple.The

latteridea(namedafterNicolasCopernicus,whoseSun-centredsystemknocked

theEarthfromitsperceivedplaceasthecentreoftheuniverse)holdsthatthere’s

nothingspecialaboutourlocation.Astronomersnoteseveralfactorsinits

favour.First,theEarthissurelyoneofmanyrockyplanetsintheuniverse.

Furthermore,ourSun,aG-typestar,isnotspecialbecausearoundoneinten

starsareGtype.Wealsoliveinahumdrumlocationinthegalaxy,alongoneof

manyspiralarms.Finally,ourgalaxyisunremarkableamongmanyinthe

observableuniverse.AsStephenHawkinghasputit,‘Thehumanraceisjusta

chemicalscumonamoderate-sizedplanet,orbitingaroundaveryaveragestarin

theoutersuburbofoneamongahundredbilliongalaxies.’

AdvocatesoftheCopernicanPrinciplealsonotehowthegreatadvancesof

sciencehighlightthefollyofassumingthatwe’respecial.Weneedthinkonlyof

Galileo’sTheStarryMessenger(1610),whichreportedhisobservationsof

moonsofJupiterandthesurfaceoftheMoon,showingthatthesebodieswere

notheavenlyperfectionsassupposedbytheologianphilosophersbutexplained

bythesamephysicsthatwehaveonEarth.Similarly,Darwin’sOriginofSpecies

(1859)overturnedtheconceitthathumansexistoutsidetherestofbiology.

TheproblemwiththeRareEarthHypothesisisthatitassumestoomuch

knowledgeabouthabitability,whereas,inreality,muchisuncertain.Recently,

forexample,it’sbeendiscoveredthatthewanderingofstarsmeansthatthe

galactichabitablezoneisalessconcreteconceptthanpreviouslythought.The

resultsofNASA’sKeplermissionalsoshowthatplanetsaroundotherstarsare

common,includingplentyofJupiter-sizebodies.Theargumentthathavinga

Jupiter-sizedplanetinjusttherightplacelowerstherateatwhichcometsor

asteroidshittheEarthhasalsobeenchallenged.Certainly,Jupitermopsupsome

impactorslikeacosmicvacuumcleaner.In1994,thecometShoemaker–Levy9

smashedintoJupiter,whilein2009and2010,thescarsoffurtherimpactswere

seenonJupiter.It’swellestablishedthatJupiterhelpstodeflectandejectcomets

thatcomefromahaloofsuchicyobjects,theOortCloud,whichsurroundsthe

SolarSystematabout50,000AUdistance.However,near-Earthasteroidsand

cometsfromwithintheplaneoftheSolarSystemrepresentmorethan75per

centofEarth’simpactthreatandJupitercanactuallydestabilizethoseobjects.

SoJupiteristwo-faced.SomecalculationssuggestthatJupiterisactuallyanet

foeratherthanfriend.

OtherargumentsforRareEartharealsoambiguous.IntheSolarSystem,Earth

uniquelyhasplatetectonics.However,forplatetectonics,aplanetmustbebig

enoughtohaveadequateinternalheattodrivetheplatemotionanditprobably

needsseawatertocooloceanicplatesandlubricatetheirmovement.IntheSolar

System,onlytheEarthqualifies.Butthatdoesn’tmeanthatEarth-like

exoplanetsinhabitablezonesmightnotalsobesuitableforsimilartectonics.

WhiletheRareEarthHypothesisiscorrectthatplanetswithoutalargemoon

willsufferlargeraxialtiltvariationsthanEarth,climaticvariationsatlow

latitudesmightbebenign.Infact,thickeratmospheres,moreextensiveoceans,

andlowerrotationratesofanexoplanetcansmooththeclimaticdifferences

betweenpoleandtropicscausedbyavaryingtilt.Finally,thequestionofoxygen

notaccumulatingonotherEarth-likeplanetsmightgointheotherdirectionto

thatassumedintheRareEarthHypothesis.Someplanetsmightbemore

favourableforoxygen-richatmospheresthanEarthbecausetheirvolcanoes

pumpoutasmallerproportionofgasesthatreactwithoxygen.Oxygenmight

buildupmoreeasily.

WhatdoesseemtobecorrectabouttheRareEarthHypothesisisthatmicrobial-

likelifeshouldbemuchmorecommonthanintelligentlife.Microbeshavea

remarkablerangeofmetabolismsandcanliveinafarwidervarietyof

environmentsthancomplexorganisms.Butanydefinitivestatementsaboutthe

prevalenceofcomplexlife—onewayortheother—simplylackdatatosupport

themandweshouldbesceptical.AsCarlSaganfamouslyremarked,‘Itpaysto

keepanopenmind,butnotsoopenthatyourbrainsfallout.’

Prospectsforastrobiologyandfindinglifeelsewhere

Theexcitementofastrobiologyisthatittriestoanswerquestionssuchasthe

originoflifeandwhetherwe’realoneintheuniverse.Withadvancesin

technology,it’sincreasinglylikelythatmajordiscoverieswillbemadeinthe

comingdecades.

Intheareaofunderstandingearlylife,it’slikelythatrealisticself-replicating

genomeswillbemadeinthelab.Thiswouldprovidegreatinsightsintothe

originoflife.SeveralgroupsarestudyingtheRNAWorldoritsvariants.There

arealsoprojectstodrilldeeplyintooldsedimentaryrocksinSouthAfricaand

Australia,whichwillsurelymakenewdiscoveriesabouttheearliestlifeand

environment.

IntheSolarSystem,Enceladusoughttobeoneofthehighestprioritiesforthe

world’sspaceagencies.Enceladushasasourceofenergy(tidalheating),organic

material,andliquidwater.That’satextbook-likelistofthosepropertiesneeded

forlife.Moreover,naturehasprovidedastrobiologistswiththeultimatefree

lunch:jetsthatspurtEnceladus’sorganicmaterialintospace.Technology

certainlyexiststobuildaspacecrafttoswingbyEnceladusandsamplethe

organicsinthejets.Betterstill,thematerialcouldbereturnedtoEarthfor

analysis.

Infact,spacecrafttocollectextraterrestrialsamplesandreturnthemtoEarth,

whicharesamplereturnmissions,arethefutureinunderstandingthehistoryof

MarsandVenusandwhethereitheroftheseplanetswasonceinhabited.A

samplereturnmissionforVenusisinthedistantfuture,butoneforMarsisa

strategicgoalofNASA’sandESA’scurrentprogrammes.

AroundJupiter,Europaisprobablythebestprospectforlife.Thefirststep

wouldbeaEuropaorbitertostudythemoonindetailanddeterminethe

thicknessoftheiceaboveasubsurfaceoceanorlakelenses.Thenextsteps

mightinvolvelanders,andpossiblyrobotstomeltthroughtheiceusing

radioactiveheatgenerators.Eventuallyoneimaginessubmarinesdivingthougha

Europanocean.

ApartfromEnceladus,TitanisatargetofastrobiologyamongstSaturn’smoons.

Ahugescientificleapcouldbemadeifalakelander—asortofinterplanetary

boat—couldfloatonthelakesinthepolarregionsofTitanandfindoutwhich

substancesmakeuptheorganicliquids.Furthermore,aTitanorbitercoulddothe

kindofreconnaissancethatwoulddeterminethedepthofTitan’ssubsurface

oceanandstudyTitan’ssurface.

Onecertaintyforthefutureisthatexoplanetdiscoverieswillcontinuetospuran

interestinastrobiology.IanticipatethediscoveryofmanyEarth-likeplanets

insidethehabitablezoneofotherstars,deadplanetswithalmostpurecarbon

dioxideatmospheres,waterworldscoveredentirelyinglintingoceans,and

youngVenus-likeplanetssweatingofftheiroceansintospacefromrunaway

greenhouseeffects.

Whenastrobiologycametotheforeasadisciplineinthe1990s,some

questioneditsfutureandwonderedifitmightbeafadthatfades,perhaps

becauseofdisappointmentinnotquicklyfindingextraterrestriallifeorafailure

toanswerquestionsaboutlife’sorigin.However,thediscoveryofEarth-sized

exoplanetsinhabitablezoneswillensurethatthepossibilityoflifeelsewhere

becomesmorerelevantthanever.Astrobiologyisheretostay.

Furtherreading

Chapter1:Whatisastrobiology?

Muchlengthierintroductionstoastrobiologyarefoundinthefollowing

textbooks:

J.O.Bennett,G.S.Shostak.LifeintheUniverse.(SanFrancisco:Pearson

Addison-Wesley,2012).

D.A.Rotheryetal.AnIntroductiontoAstrobiology.(Cambridge:Cambridge

UniversityPress,2011).

K.W.Plaxco,M.Gross.Astrobiology:ABriefIntroduction.(Baltimore:Johns

HopkinsUniversityPress,2011).

J.I.Lunine.Astrobiology:AMultidisciplinaryApproach.(SanFranciso:Pearson

AddisonWesley,2005).

W.T.Sullivan,J.A.Baross(eds).PlanetsandLife:TheEmergingScienceof

Astrobiology.(Cambridge:CambridgeUniversityPress,2007).

Thedevelopmentofastrobiologyfromthe1950sonwardsisdescribedby:S.J.

Dick,J.E.Strick.TheLivingUniverse:NASAandtheDevelopmentof

Astrobiology.(NewBrunswick,NJ:RutgersUniversityPress,2004).

Anoldclassiconthenatureoflifeis:E.Schrödinger.WhatIsLife?(1944;

Cambridge:CambridgeUniversityPress,2012).

Chapter2:Fromstardusttoplanets,theabodesfor

life

AreadablediscussionofmodernBigBangtheoryisgivenby:C.Lineweaver,T.

Davis.2005.MisconceptionsabouttheBigBang.ScientificAmerican292:

36–45.

ApopularaccountofArthurHolmes’squesttofindtheageoftheEarthis:C.

Lewis.TheDatingGame:OneMan’sSearchfortheAgeoftheEarth.

(Cambridge:CambridgeUniversityPress,2012).

Chapter3:Originsoflifeandenvironment

Thescienceoftheoriginoflifeisdescribedby:R.M.Hazen.Genesis:The

ScientificQuestforLife’sOrigin.(Washington,DC:JosephHenryPress,

2005).

AluciddescriptionoftheearlyevolutionoflifeonEarthis:A.H.Knoll.Lifeon

aYoungPlanet:TheFirstThreeBillionYearsofEvolutiononEarth.

(Princeton:PrincetonUniversityPress,2003).

Chapter4:Fromslimetothesublime

TheEarth’sformation,evolution,andhabitabilityarecoveredin:C.H.

Langmuir,W.S.Broecker.HowtoBuildaHabitablePlanet:TheStoryof

EarthfromtheBigBangtoHumankind.(Princeton:PrincetonUniversity

Press,2012).

Chapter5:Life:agenome’swayofmakingmoreand

fittergenomes

Awidelyusedintroductorytextbookonmodernmicrobiologyis:M.T.Madigan

etal.BrockBiologyofMicroorganisms.(SanFrancisco:BenjaminCummings,

2012).

TheeffectsoflifeontheEarth’schemistryonagloballevelaredescribedinthe

followingtextbook:W.H.Schlesinger,E.S.Bernhardt.Biogeochemistry,

ThirdEdition:AnAnalysisofGlobalChange.(SanDiego:AcademicPress,

2013).

Chapter6:LifeintheSolarSystem

TheplanetsoftheSolarSystemandtheirhabitabilityaredescribedinthe

followingtextbook:J.J.Lissauer,I.dePater.FundamentalPlanetaryScience:

Physics,ChemistryandHabitability.(Cambridge:CambridgeUniversity

Press,2013).

Chapter7:Far-offworlds,distantsuns

Areadablebookthatdiscussesthesearchforhabitableexoplanetsis:J.F.

Kasting.HowtoFindaHabitablePlanet.(Princeton:PrincetonUniversity

Press,2010).

Chapter8:Controversiesandprospects

Thecontroversialbutengrossingbookthatarguesforthescarcityofcomplex

lifeis:P.D.Ward,D.Brownlee.RareEarth:WhyComplexLifeisUncommon

intheUniverse.(NewYork:Copernicus,2000).

Index
A
acetyleneonTitan106
adaptiveoptics113
aeonsonMars90
ageoftheEarth25–6
albedooftheEarth56
Aldebaran(star)18,20
ALH84001meteorite98–9
Alpha-CentauriB112
alterationminerals93–4
aluminium-26atoms22–3
AmesResearchCenter1
aminoacids76
ammonia(NH
3
)12,107–9
Andromedagalaxy15
anti-biosignatures119
ApexChertrockformation,Australia42
archaea66–9,76,78,80
Archaeanaeon32,41,46–8,50–1
Aristotle3
asteroidimpacts2,24,25,30,61–2,96,126(seealsometeorites)
astrometry111
atmosphere84–5

onEarth28,32–4,44–53,54,55,118–19,125
onexoplanets59,117–19
onMars84,85,89–90,94–6,97
onPluto108
onTitan(moonofSaturn)103–4
onTriton(moonofNeptune)107
onVenus84,85–8
atomscommontolife9–10
ATP(adenosinetriphosphate)35
B
bacteria66–9,75–6,78,98(seealsocyanobacteria)
survivalinextremeheat79–80
bandedironformations50
Bernal,J.Desmond6
Betelgeuse19
BigBang14,16–17
bioastronomy5–6
biomarkers42–3,54
biomass49,63–4,102,108
biomolecules70–1,73
chirality37–9
biosignatures118–19
biospheres60,63–5,115,121
blackdwarfs19
blackholes20
bodystructures60
‘boringbillion’53–5
brainmass,proportional122–3
Brasier,Martin42
Brock,Thomas79–80
Brownlee,Don125
Budyko,Mikhail56

Buick,Roger40
C
Callisto(moonofJupiter)102
CambrianExplosion54,59,60
capcarbonates57
carbohydrates71,78
carbon,organic10,33–6,39–40,42,50–52,55,74
carbonassimilationexperiment96
carbon-basedlife9–10
carbondioxide11,32,34
intheEarth’satmosphere45–7,51,57
greenhouseeffect45–8,85–6,95,116
inthehabitablezone117,129
onMars84,85,89,94–6
onVenus84,85,86
carbonate–silicatecycle47–8
Cassini–Huygensmission104–5
cells58,65–70,78
circulation8
Ceres(largestasteroid)83,99
Charon(moonofPluto)108
chemicalelements65
chemicalweathering93
chemiosmosis35
chemoheterotrophs74–5
Chicxulub,Mexico61–2
chirality37–9
Chiron29–30
chlorine58–9
chromosomes69–70
Chyba,Christopher9
Cleland,Carol9

climate
onMars94–6
regulation47–8
CO
2
seecarbondioxide
Cocconi,Guiseppe120
coherentenergy6–7
continents,formation28–9
continuouslyhabitablezone(CHZ)115
convergentevolution122
CopernicanPrinciple125–6
coronograph113
CosmicConnection(Sagan)13
CosmicMicrowaveBackground16–17
cosmobiology6
Cosmotheoros(Huygens)4
Creataceous–Paleogenemassextinction61–2
Crick,Francis72
cryovolcanism107–8
CuriosityRover89–90
cyanobacteria42–3,49–50,68–9,76
D
Darwin,Charles8–9,31,126
Dawnmission99
definitionofastrobiology1–2,5
Democritus3
densityofexoplanets114–15
directdetectionofexoplanets113–15
dissipativestructures8
DNA32–3,36,69–70,71–3
DNAsequencing77–8
Dopplershift111–12
Drake,Frank(DrakeEquation)120–22
dropstones56

dustparticles10
dwarfs19–21,106,116–17
E
Earth84
age25–6
atmosphere28,32–4,44–53,54,55,118–19,125
basisoflife9,12,63–70
earliestaeon28–36
developmentofintelligentlife122,124
massextinctions60–2
originoflife1–2,31–43
asa‘PaleBlueDot’118
position14–15,26–7
SnowballEarthhypothesis55–8
uniqueness3–4,125–7
Ediacarans54
Einstein,Albert113
elements
chemical65
commontolife9–10,89
isotopes22–3,25–6
non-metallic12
instars17–18,19,21,117
enantiomers38–9
Enceladus(moonofSaturn)103,128
EncephalizationQuotient(EQ)122–3
endosymbiosis69
energy6–9,87,106
infrared45
life-giving65
metabolic35,58,73–5

inphotons18
precursorforlife58
entropy6–8
eukaryotes59,66–70,76,78,80
Europa(moonofJupiter)100–2,128–9
iceon2–3
EuropeanSpaceAgency114
evolution8–9,48,75
during‘boringbillion’53–5
chemical31
convergent122
diversityin60
exobiology5
exoplanets3,24–5
atmosphereon59,117–19
detection110–15
evidenceoflifeon115–19
intelligentlifeon120–4
expansionoftheuniverse16–17
extinctions,mass60–2
extraterrestrialintelligence(SETI)120–4
extraterrestriallifeevidenceof2–3,4
intelligent120–4
likelihoodofcarbonbase9–10
likelihoodofsiliconbase10–11
probability16
significance12–13
theoriesabout3–4
extremophiles79–81
F
faintyoungSunparadox44–5

Fermi,Enrico(FermiParadox)123–4
fluorine58–9
fossils41–2,53–4,98
G
G-typestars125–6
galacticfilaments16
galactichabitablezone(GHZ)15,117–18,126
galaxies15–16
birthof17
GalileanmoonsofJupiter99–102
Galileo126
Galileospacecraft2–3,101,118
gametes69–70
Ganymede(moonofJupiter)102
gaschromatographmassspectrometer(Vikinglander)97
gasexchangeexperiment(Vikinglander)96–7
gasgiants23–4
genetransfers77,79
genetics75–9
genomes8,36–7
giantimpacthypothesis26
giantplanets23–4
glaciations55–8
GreatOxidationEvent48–53
greenhouseeffect45–8,85–6,116
onMars95
gulliesonMars91
H
habitablezone(HZ)115–17
Hadeanaeon28–36

Haldane,J.B.S.32
half-life22,25
Hawking,Stephen126
helium17–18,20
Herrmann,Joachim5
Hertzsprung–Russell(H–R)diagram20–21
Holmes,Arthur25–6
homochirality39
Hooker,Joseph31
hotJupiters24–5,110,112
Hubble,Edwin16
Huntress,Wes5
Huygens,Christiaan4,104–5
hydrocarbonsonTitan106
hydrogen17,20,52–3,88
hydrogenbombs18
hydrogenperoxide90
hydrothermalvents34–5,42
I
ice
density11–12
onEuropa(moonofJupiter)2–3
formation55–7
icegiants23–4
impacterosion96
impactswithEarth28–30,33–4,61–2
indirectdetectionofexoplanets110–13
infraredradiation45–6,87,93
innerplanets,wateron84
intelligentlifeonexoplanets120–4
interferometry114
intraterrestriallife64
Io(moonofJupiter)100

iron50,51
iron-60
atoms23
isotopes22–3
radioactive25–6
Isua,Greenland39–40
J
Jupiter65,85
formation24
moons2–3,99–102,128–9
orbit111
asprotectionforEarth126–7
resonancewithSaturn30
K
Kstars116
Kant,Immanuel4,22
Kelvinscale20
Kepler,Johannes3–4
Keplermission112–13,126
Kirschvink,Joe57
Klein,Harold‘Chuck’97
KuiperBeltobjects(KBOs)107,108
L
labelledreleaseexperiment97
Lafleur,Laurence4–5
LakeVostok,Antarctica80–1
Laplace,Simon-Pierre(MarquisdeLaplace)22

Laplaceresonance100
LateHeavyBombardment30,39
leadisotopes,age25–6
Lederberg,Joshua5
life
characteristics6–9
originonEarth31–43
precursors58–9,65
light
bending113
cancelling114
lightyears15
lightning,creationoforganicmolecules32
Linnaeus,Carolus66
lipids71
liquidwater10,11–12,65,82–6,89,90–6,99–103,106,115–16,119
LocalGroup15
Lowell,Percival4
M
Mdwarfs116–17
MacGregor,Alexander32
magnesium-26atoms22–3
magneticfield56
onEuropa101
magnetite56,98–9
mainsequence20–1,44,115–16
MarinermissionstoMars88–9
Mars84–6,88–99
inthehabitablezone116
intelligentlife4
signsofancientlife2
MarsExplorationRovers94

MarsScienceLaboratoryseeCuriosityRover
massextinctions60–2
Mayor,Michel111–12
mediumforbiochemicalprocesses11
Mercury84–5
messengerRNA(mRNA)73
metabolism35,58,73–5
metallicityofstars117–18
meteorites25,33–4(seealsoasteroidimpacts)
age26
fromMars97–9
methane55,90
atmospheric46–7
onTitan104–6
Methanopyruskandleri80
Metrodorus3
microbes64–5,127
extraterrestrial2
genetransfer77,79
survivalinextremeheat29,79–80
microbialconjugation69
microbialmats40–1
microfossils41–2,98
microlensing113
MilkyWaygalaxy15,16
Miller,Stanley32–3
Mitchell,Peter35
molecularclocks78–9
molecules,organic42–3
Moon14
craters30
formation26–7,28
Morrison,Philip120
Murchisonmeteorite33

N
NASA1,2,5,88–9,99,101,112,114,118,126
naturalselection8,75
Nazcaplate,SouthPacific47
nebularhypothesis22–3,25
negativeentropy7
Neoproterozoicglaciations55,56,57
Neptune
distance15
formation24
moons106–8
orbits30
neutrons19
Nicemodel30
non-metallicelements12
NorthPole,Australia40–1
nuclearfusion17–18,19,21
nucleotides71–3
O
oceanicplates47
OfthePluralityofWorlds(Whewell)4
OortCloud126
Oparin,Alexander32
organiccarbon10,33–6,39–40,42,50–2,55,74
organicmolecules32–6
originoflifeonEarth31–43
OrionArmoftheMilkyWay15
outflowchannelsonMars91–3
oxygen32
onEuropa102
onexoplanets127
levels48–53,54,55

precursorforlife58–9
oxygenationtime59
ozonelayer52
P
PAHs(polycyclicaromatichydrocarbons)98
‘PaleBlueDot’118
Paleaoproterozoicglaciations55,56,57
panspermia31
Patterson,Clair26
Pauling,Linus7,78
PCR(polymerasechainreaction)technology80
Pelagibacterubique64–5
Permian–Triassicmassextinction61
PhanerozoicAeon54
photonsintheSun18
photosynthesis49–51
phylogeny77–9
Pikaia54
planetaryembryos24
planetarymigration25
planetarynebulae18–19
planetesimals24
platetectonics48,127
Plato3
pluralism3
Pluto108–9
primevallead26
primordialsoup32
prokaryotes67–8
proteinsynthesis77
proteins71,76
ProterozoicAeon55
protongradient35

ProximaCentauri15
Q
Queloz,Didier111–12
R
racemicmixture38
radialvelocitymethod111
radioactiveisotopes25–6
radiogenicheat101
RareEarthHypothesis125–7
recombination70
reddwarfs21
redgiants18
redsupergiants19
redoxtitration53
reductants52
ribosomes68
inRNA(rRNA)77
RNA36–7,39,71–3,77
rockyplanets21,23
runawaygreenhouseeffectonVenus86–7
runawaylimit87–8
S
Sagan,Carl13,14,127
samplereturnmissions128–9
Saturn
formation24
moons82,102–6,129

orbit111
resonancewithJupiter30
scablands93
scaleoftheuniverse14–16
Schiaparelli,Giovanni4
Schneider,Eric8
Schopf,Bill42
Schrödinger,Erwin7,8
SecondLawofThermodynamics7
SecretService1
sedimentaryrocks39–41,50
SETI(searchforextraterrestrialintelligence)120–4
sexualreproductionearly54
ofeukaryotes69–70
Shapley,Harlow115
siliconbasedlife10–11
Simpson,GeorgeGaylord5
sizeofMars96
SnowballEarthhypothesis55–8
socialanimals123
soilonMars96–7
solarflux85,87
SolarSystem15
age25–6
formation22–5
planetaryorbits30–1
possibleabodesoflife83–109,128–9
SouthAfrica,microfossils42
species75–6
spectraofexoplanets119
spectralclass21
SPONCHelements12
stablemediumforbiochemicalprocesses11
stars,lifecycle18–21
StrelleyPoolFormation,Australia42

stromatolites40–1,50
Struve,Otto5
subduction47
subsurfacebiosphere64
sulphatesonMars94
sulphur51–2
sulphurdioxideonMars95
Sun14–15,44–5
elementsin17–18
lifecycle18–19,21
Super-Earths110
supernovae19–20,21,23
surfacestability65
symmetryseechirality
synchronousrotation116–17
T
taxonomiclevels66
technosignatures120–4
telescopes,space-based113–14
Thales3
Theia,impactwithEarth26
TheoryofGeneralRelativity113
thermodynamics6–8
thermophiles29–30,79–81
Thermusaquaticus80
tidallocking116–17
Tikov,Gavriil5
tillites56
tilt127
ofMars95
Titan(moonofSaturn)82,103–6,129
transcription73
transitmethod112–13,119

transittimingvariations114
TransitingExoplanetSurveySatellite(TESS)113
transmittingcivilizations120–4
treeoflife76,77
Triton(moonofNeptune)106–8
TumbianaFormation,Australia50
U
universe
expansion16–17
size14–16
uraniumisotopes,age25
Uranus
formation24
orbits30
Urey,Harold32–3
V
valleynetworksonMars91–2
Venus84–8
Vikinglanders9,89,96–7
Virgosupercluster15
viruses70
volcanoes33,57
onIo100
causeofmassextinction61
Voyager1spacecraft118
W
Ward,Peter125

water,liquid10,11–12,65,82–6,89,90–6,99–103,106,115–16,119
WaterbeltEarth57–8
Watson,James72
weirdlife82,106
Whewell,William4,115
whitedwarfs19
wobblesinstellarorbits111
Woese,Carl76–7
Z
zircons29
Zuckerkandl,Emile78

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GALAXIES

AVeryShortIntroduction

JohnGribbin

GalaxiesarethebuildingblocksoftheUniverse:standinglikeislandsinspace,

eachismadeupofmanyhundredsofmillionsofstarsinwhichthechemical

elementsaremade,aroundwhichplanetsform,andwhereonatleastoneof

thoseplanetsintelligentlifehasemerged.InthisVeryShortIntroduction,

renownedsciencewriterJohnGribbindescribestheextraordinarythingsthat

astronomersarelearningaboutgalaxies,andexplainshowthiscanshedlighton

theoriginsandstructureoftheUniverse.

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PLANETS

AVeryShortIntroduction

DavidA.Rothery

ThisVeryShortIntroductionlooksdeepintospaceanddescribestheworldsthat

makeupourSolarSystem:terrestrialplanets,giantplanets,dwarfplanetsand

variousotherobjectssuchassatellites(moons),asteroidsandTrans-Neptunian

objects.Itconsidershowourknowledgehasadvancedoverthecenturies,and

howithasexpandedatagrowingrateinrecentyears.DavidA.Rotherygives

anoverviewoftheorigin,nature,andevolutionofourSolarSystem,including

thecontroversialissuesofwhatqualifiesasaplanet,andwhatconditionsare

requiredforaplanetarybodytobehabitablebylife.Helooksatrockyplanets

andtheMoon,giantplanetsandtheirsatellites,andhowthesurfaceshavebeen

sculptedbygeology,weather,andimpacts.

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STARS

AVERYSHORTINTRODUCTION

AndrewKing

Everyatomofourbodieshasbeenpartofastar.Ourveryownstar,theSun,is

crucialtothedevelopmentandsustainabilityoflifeonEarth.ThisVeryShort

Introductionpresentsamodern,authoritativeexaminationofhowstarslive,

producingallthechemicalelementsbeyondhelium,andhowtheydie,

sometimesspectacularly,toendasremnantssuchasblackholes.

AndrewKingshowshowunderstandingthestarsiskeytounderstandingthe

galaxiestheyinhabit,andthusthehistoryofourentireUniverse,aswellasthe

existenceofplanetslikeourown.Kingpresentsafascinatingexplorationofthe

scienceofstars,fromthemechanismsthatallowstarstoformandtheprocesses

thatallowthemtoshine,aswellastheresultsoftheirinevitabledeath.